THINGS WRONG/ODD
(for reference, the usage of ellipses after or before quotation marks is to denote that a quote either has more text before the part that's being focused on, or after. in-between means there's some amount of text in between two other pieces of text that is being skipped over for brevity.)
"This room unlike the room below was furnished." - should be "This room, unlike the room below, was furnished."
"his voice nasely" - "nasely" should be "nasally"
"The closer they came the more clear his features became" - not necessarily wrong, but clunky sentence structure due to usage of words "came" and "became"
"over sized" - should be "oversized"
"And on his face was plastered a smug grin" - starting a sentence with "and" is not necessarily incorrect, however it makes the flow of this sentence slightly odd towards the beginning and doesn't flow well with the sentence prior to it
"higher ups" - should be "higher-ups", with a hyphen.
"Personally rags hated boots," - "rags" is uncapitalized despite having been established as a character's name, therefore making it a proper noun in need of capitalization
"The fox raised his hand and pointed to a small shed "There is the storage room...." - missing comma or colon before the start of a quote
""In this tent is the message, there we eat the food."" - "message" believed to be incorrect substitution for "mess hall", a word for a designated room for eating typically used in military contexts
"cavalryman's gloves" - should be "cavalrymen's gloves"
"forseeable" - should be "foreseeable"
""And last, here is the barracks. This is where you will sleep for the forseeable future."" ... ""And last but most certainly not least, your commander, myself. Reynault."" - starting multiple sentences with "And last" in close proximity to each other feels like bad flow, but not technically incorrect
"The Kelpy" - capitalization of "Kelpy" is inconsistent with previous instance of lowercase "kelpy" earlier in the text. species names are not proper nouns and generally do not require capitalization when not used at the beginning of a sentence
"he one thing he couldn't stop talking about was he hair and how great it was." - should be "his hair"
"Between that and the unpacking time went by quickly" - missing comma between "unpacking" and "time"
"...everywhere ya go?" Redrake asked in disbelief." - missing comma before the end of a quote (will refrain from listing every instance of this particular error for the sake of length)
"...followed by the shouting of a nasally voice"..."Attention recruits, report to the courtyard at once." - ending a quote with a period when said quote is meant to be perceived as being "shouted" gives an inconsistent idea as to the tone of the quote itself
"None of them were told too," - should be "to"
"niether are we part of the Royal Guard." - should be "neither"
"monster-kind" - "monsterkind" is not a hyphenated word in UT
"soft locking" - should be "softlocking"
"By the end of the day, they were exausted frustrated, and confused" - somehow included the oxford comma before the conjunction "and", but forgot the comma between "exausted" (which is spelled wrong) and "frustrated"
"dry dusty embrace of sleep" - missing comma between "dry" and "dusty"
"instructer" - should be "instructor"
"the eponymous instructor began" - incorrect usage of "eponymous", a proper word would be "aforementioned"
"...then you place in the slot in the back like so, and charge it."" - should be "then you place it in the slot..."
"Ah, monsier Rags." - inconsistent capitalization of "Monsieur". also spelled wrong
""Oh." Was all Rags could say" - improper capitalization of the word immediately after a quote
"I'm still trying to break it like you suggested..." - should be "break it in", unless we're meant to believe he is literally trying to destroy the prosthetic
THE BIT YOU'RE HERE FOR, PROBABLY
(this will be ordered according to how I see fit to write it, so expect topics to jump a bit)
Overall, I think the general tone of the work is good. It's balance of seriousness and light-heartedness works well with the style typically associated with UTDR, and the more notable fanworks within the UTDR sphere. Characterization I found was slightly hit-or-miss, with Reynault having a much stronger and more well-defined personality than Erin, for example. The protagonists themselves, Rags and Goz, didn't stand out as being anything particularly special, but they were fine for the purposes of this story. Generally speaking, this was a fairly decent piece of creative writing, and a nice little story about this military company within the UT universe.
There were some instances (about two of which I believe I noted in the list above) where the flow of certain sentences was made noticeably clunky due to some odd word positioning or word choice, such as when multiple sentences began with "And last" in close proximity to each other, it made things feel less smooth than they otherwise could have been. I also found a few certain examples of punctuation seeming to clash with the intended tone or reading of a given quote, such as when something that was meant to be shouted ended with a period rather than an exclamation mark. While not necessarily incorrect, it seems to make the actual thing that's being said have a slightly odd tone to it. It reminds me of that joke about how adding a tilde (~) to the end of a sentence immediately makes it "softer", it changes the tone.
I found the various little tidbits about Rags, such as him not liking being moist, to be fairly interesting, and I feel like those sorts of things genuinely do fit alongside the other weird little details monsters seem to have in the UT universe, such as Toriel having fur in her sink's drain, or Grillby also being unwilling to touch water (though for more obvious reasons).
However, this does lead me to one of my larger specific problems with one of the pieces of lore this story establishes.
The scene where Rags panics after believing himself to have consumed chocolate is humorous, and a decent insight into a small piece of his past, but this is potentially incompatible with the way UT treats monsters in it's setting, given how various animal-like monsters can be seen consuming things that would be lethal to their real-world counterparts, such as that rabbit in Grillby's appearing to be inebriated despite alcohol being highly lethal to rabbits (and most common animals, such as canines) even in small concentrations, which would seem to imply that animal-like monsters in the world of UT do not possess the same vulnerability to certain compounds exhibited by their real-world counterparts, meaning that a canine-monster could very likely consume chocolate or any other normally poisonous compounds (for canines) with no problems in the UT universe.
I found the level of technology depicted to be interesting, reminding me vaguely of Arcanum: Of Steamworks and Magick Obscura, though I find the actual level of technological advancement of monster civilization to be somewhat questionable, given that the underground as we see it in UT seems to largely be pre-industrialization in most aspects, save for a few certain areas.
I also find the mechanical mimic's ability to detect the strength of one's soul to be questionable too, given both that the majority of actual in-universe research about the soul seems to have been conducted in the lab in Hotland by either Alphys or Gaster, and that the actual power of the soul, "determination", isn't necessarily actually tied to just one's soul when it comes to monsters, since monster's bodies and souls are much more intertwined than human's are, meaning a mechanical mimic could theoretically go off on a particularly determined monster as well as a human.
I guess maybe it's possible that whatever soul detection mechanism is used by the mechanical mimic isn't really the most precise instrument in the world and it just relies on the presumably massive difference in strength between monster souls and human souls, but it still just strikes me as somewhat implausible that monster civilization would have somehow developed such a technology and not put it to use in other circumstances, or at least held on to the knowledge of how to produce this technology for use in the future, while in UT it seems that exactly one person in the entire underground is capable of detecting the actual strength of a human soul directly. Basically, it just seems like such a specific and important ability wouldn't logically exist at this point in time, even mechanically.
Alright, that's most of what I have to say on this story at the moment. I'm sure there are some more specific things I could say about it if you prompt me to sometime later in this thread or others, but right now I'm spent so I'm gonna get back to the other thing I'm supposed to be doing.
You did some good stuff with this one, I hope it's not too long before we get to see more of Oldentale.