How is the Allagan equipment explained in canon?

[Endemerrin]

composed of the 5 playable races or more.

That's the thing, though. I mention it in my OP, but both the Hyur and Miqo'te didn't migrate to Eorzea until thousands of years after the fall of the Allagan Empire. It just doesn't make sense for there to be equipment tailored to fit the Miqo'te specifically. Given that we known Darnus is of an Allagan bloodline, I feel as though the Allagans may have been a precursor to modern day Elezen. Suffice it to say, while Roegadyn and Lalafell technically could have been a part of the empire, I don't think there would be any Miqo'te that lived in the empire unless there were some groups that migrated prior to the ice age.

[Urthdigger]

It's possible we can use dark matter not just to repair gear, but modify it as well. Such as size alterations or adding holes. It's just not reflected in-game that we carry grade 0 dark matter around for that.

[Velox]

Acheron is an Allagan, or rather, his facade is (being "enhanced" of course through technology). He bears a striking resemblance to a Hyur, but we know through the lore that he can't be a Hyur. With this, I believe that the Allagans were their own race. We can look at Garleans as an example. There seems to be 2 types of Garleans, ones who join the empire (any race) and pure-blood Garleans (Cid for example). Being pure-blood signifies not tainted by mixing with another race, so it's safe to assume that Garlean is its own race. So even though Allagans and true Garleans bear resemblances to Hyur, they do have their own race-specific identities. For Garleans, it is the third eye. For Allagans, it seems to be odd-colored skin tones.

If you look at THIS wallpaper of future CT bosses, you can see the similarities in the humanoid ones in that they all share odd facial markings. Of course, this could be war paint, but we can't rule out this being a racial trait as well.

This is just me speculating though.

As for Miqo'te tail holes...I'm pretty sure all Miqo'te carry around a pair of trusty scissors and do a quick mend right before equipping the armor :P

[Lollie]

Don't even need scissors, we have claws ya know.