[Aerich]

Slings are no advantage over bows against skeletal undead. Both slings and bows cause missile damage, thus doing only half damage to skeletons. Slings will do more damage if you have exceptional (18/xx) Strength, but bows give a higher ApR.

There's no need to cap the mages in your party for any reason other than number of spell scrolls. I'm running a party right now with three multiclass mages, and it's doing fine. It's unnecessary to switch mage casters in and out for clerics. Patience and good strategy are the keys to starting fresh characters on HoF. As long as you have spellcasting classes covered for later on in the game, your party composition doesn't matter too much. The orc caves can be completed without summons. You won't get summons in time for the orc cave unless you are using single-class clerics.