As you probably realise, I have thought long and hard about this subject. The two most distinctive things on a 28mm miniature are going to be the helmet and the size of the shield, until the later period when the length of the hauberk becomes obvious.
So when you are talking about unarmoured warriors (and IMO they should be the majority) most or all figures from MM, GB, Foundry and OG (yes I went there!) will suit, although like you say, the size of the shield stands out between early and mid period.
For armoured warriors, MM, GB (Mid/Late) and most of the Foundry will suit your purposes.
Or another way to look at it is avoid long hauberks, kite shields and conical helmets.