First of all, welcome to GameBanshee forums, Lizard (if you don't me calling you that for short).
Now, about your questions...
1) Normally, you can't dual-class into a sorceror w/ the normal game (including the expansion) OR with the patch installed. As I honestly am unfamiliar with the mods, I don't know which one would allow that. Just thought I'd clear part of the matter.
2) Also, normally you can't dual to kits (except w/ a mod for the Shadows's of Amn portion of the game which I can't recall the name of), but if you want to dual to a mage, you have to have 17 Int at the least. Quite a stretch, isn't it?
3) That cleric of yours practically sounds best on it's own. If you give him the right High-Level Abilities (those bonus spells you can learn passed level 22), and cast the right buff spells on him (depends upon the situation), he can last pretty well in the long run.
Hope this all helps. Good luck.