Front Rank figures generally mix well with Calpe and Elite.
They're also OK with Perry but the styles are different, i.e. FR are chunkier, and some of the Perry figures are, like Foundry, sometimes noticeably smaller even within a range e.g. the Perry Prussian gunners are on the small side.
But you can successfully have them all on a table at the same time as I do at the moment and they look fine!
Cheers,
Paul