Being completely historically correct with Playmobil is an issue I've been wrestling with for quite some time, and while I'd like to be as accurate as possible, sometimes it's easier and more fun not to be.
For example, my Egyptian army includes cavalry, which the ancient Egyptians didn't really have, and some of the klickies I'm using for their enemies have iron armor and weapons, which wouldn't have been historically correct either. Also, I'm using my Egyptian army to fight my Romans but I'm calling them Meroians, which would have been historically correct, except that Meroians would have had iron arms and armor, but mine don't. I've decided I can live with these anachronisms.
My French army and redcoat officers are basically historically correct, but my redcoat soldiers are not. I'm trying to represent the period from the Seven Years War through the American Revolution, but the redcoat soldier's double-breasted tunics are from the Napoleonic period. I can live with this, but if the new redcoat blister pack materializes, I'll probably replace the double-breasted soldiers with them.
Like others have mentioned, Medieval fighting men from the early 12th Century are quite different from their early 15th Century counterparts. While I think all my Medieval men look pretty good together, if one looks closely and knows armorial evolution, there are some major problems with my klickies, as I have Crusaders wearing chain mail and Norman helmets holding classic shields next to knights wearing full plate armor and basinets holding Norman shields! I try not to get too bothered by it.
With my Romans, I'm fortunate in that they could work from a little before 40 AD until almost 200 AD, which is the period I'm aiming for.