I've got Foundry units made up from Mexicans from the Old West Range - what was until recent the 'South of the Border' collection - so figures from Foundry packs OW162, 163, 164, 165, 167, 168
https://www.wargamesfoundry.com/collections/old-west?page=6Those are all 25mm rather than 28mm. They're fine in seperate units, and nice enough figures but definitely not mix and match with Gringos (even taking into account that not all people are the same size and height!). Some were so small by comparison that I just got rid of them. Although strangely, the 'Capt Zacarias' figure in OW167 Mexican Soldiers Command is actually pretty similar in size to some of Gringos.
I haven't yet tried any from what was their Mexican Adventure Collection: OW171 - OW178 - don't know if there is a size difference there, but I think those are the sets people have said are closer to 28s.
They've just rereleased their original Maximilian Adventure range too, which is what I think you're referring to? In a seperate category from the Old West range - haven't yet got any of those but I may well do, even if for seperate units, as they've got things like Imperial Infantry and suchlike.
Upshot being: Foundry figures are nice, size varies and mostly too small to mix with Gringos, but will do in seperate units.
Gringo's are excellent, lovely to paint, I mean really lovely, and worth waiting for expansions to the range

(But in the meantime I'll happily paint some smaller chaps too)