[Radek]

Mixing armor does not make any harm but you must be good in all armor skills used by your pieces of armor.
As to the medium armor, yes it sucks in Morrowind. There exist only one piece of medium armor in (plain) Morrowind, which is worth of mentioning (cuirass, Ebony Mail) but you get this cuirass only if you become Archpriest in the Temple. Let us summarize:
- the game contains Light Armor (glass armor) with better armor rating than any medium armor (except Ebony Mail), glass armor is accessible easily (a full set can be even bought).
- heavy armor is, as it were, the best armor in the game but it is super heavy. You will need a big STR boost if you want to wear it. STR 100 is far from sufficient.