I concur, that they'll mix fine when on the table/within a unit.
Just a point re which medieval ranges you are referring to:
- Perry Wars of Roses/European Armies and Foundry Late Medieval are fine to mix
- Perry Agincourt to Orleans and Foundry HYW are Ok to mix (I have done so), but the latter range is about 50 years earlier in terms of style of armour & dress.
Good luck with the figs.
Simon.