When did the Imperial Legion start recruiting non-Imperials to fight side by side with them? Were they ever a homogenous Army? Did non-Imperials fight in seperate formations, or were they mixed in from the start?

As far as we know, the Legion has never restricted membership to the Legion and actual detail about the inner workings of the Legion havn't been revealed, other than a mention of them using the phalanx during the invasion of Hammerfell in Vivecs Sword Swinging Meeting with Cyrus the Restless.

In morrowind you can join the legion regardless of your race, and they have A LOT of orcish members. That can't have been the case for longer than Daggerfall, but still.

They probably start using other races a few years after they're conquered.

P.S They're not technically "foreign" they're all part of the same nation so...