I would recommend all the same species as it'll be the most natural for them, but most of them learn to mix, I think. Typically, the recommendation is that you get at least 6 for a group, but in a 75 you could get a lot more than that. It also depends on which ones you get. If you are talking about the dwarf corys, then you could get a LOT (like 15-20).
You can definitely mix them if you wanted. But yea, getting a nice school of the same species would be nice. I would probably just get a nice school of about 6 or 7 total if they are the full sized ones. If you already have mixed species then just get about 3 more of a kind you already have.