Like you say, it's fantasy, so you can do whatever the hell you want.
I can't think of a game that's been seriously criticized for "mixing eras." Final Fantasy 7 mixes sci-fi, medieval, feudal, and a few others. Chrono Trigger (which, to be fair, is a game about time travel) has eras where swords & sorcery interact with flying ships and steamboats and electronics. The list goes on... there are probably actually pretty few RPGs that stay confined to a single era.
And if you do want to maintain some self-consistency, what everyone here is saying about different wealth levels is true enough. Plus you also have geographical issues that play into socio-political issues. Europe and America were in vastly different spaces during the medieval era - they've converged a bit more recently now that we have instant communication and fast travel options, but that hasn't always been true.