I've been using very mixed up suits on my stealther with no fail, pieces including plate gloves and tunic, studded arms, hide arms/legs, etc...

... but what I think the gimmick is, is that mage armor behaves like leather no matter what material it actually is. That, and/or lower requirements (which my plate chest had) clearly affect stealth.

Sadly, there's no actual formula to figure these things out.