I'm more of a fan of the "Class and a Half" advancement scheme for multiclassing as compared to straight mutilclassing.

And suggestions on restricted class combos?

Plenty -- no paladin or monk multiclass with anything; multiclass assassin only with fighter, cleric, and illusionist; class AD&D racial allowances, etc etc.

I guess it could depend on the setting, though, so you could allow a variety of stuff if you wanted.

Opps! I meant Class and Half for AA, not C&C in general.

So perhaps you should only have the Rogue style PC (can't remember the name) should only support a Wisdom and Charisma based Arcanist.

And Pugilists can't multiclass with anything.

My own recommendation would be for Pugilists and Arcanists not to be able to multiclass with anyone, as these two classes require a lifetime of devotion on the part of the practitioner. Anything else would be fair game.