I would honestly say newer players should play stronger countries since it's more forgiving of failure. Even taking one unlawful province in the HRE land you with -120 relations with other HRE states and can get you an easy rolling. It's not newb friendly by any means. If we have major powers as players, I would just put a basic rule of Great Powers not to ally each other or at most one other great power ally.
A few good middle powers that aren't in too bad a position outside the HRE in European/North Africa/Mideast include: Portugal, Oman, Ethiopia, Hungary, Tunisia, Hejaz, Morocco, and Scotland. Inside the HRE some good choices for middle/small powers Brandenburg (the classic), Bohemia, Hanseatic League, Palatinate, Bavaria, Switzerland, and Hesse.
I'd recommend Castile and Ottomans for those completely new.
I don't have Mare Nostrum but the changes makes the Barbary States of North Africa pretty appealing to me.