I'm not much of a one to ask about 15's but I think you will be able to get away with quite a bit as regards armours etc as well, they are not going to be that visible.
Clothing wise that might be more obvious so I'd stick to Early HYW ranges (obvious I know).
I'll leave it up to someone who knows their 15's to comment any further

Oh, Martin at Vexillia would be the man to ask about Medieval 15's for any period.