Knights, to no one's surprise, are great at smashing things and can't do much else. I would suggest aiming your early Expert picks at your choice of Spear or Sword, plus Plate Armor, Armsmaster, and Bow. Pick up Repair Item too, it is a nice skill to have and Knights have little else for utility.

Thieves, on the other hand, have tons of utility. They're not half bad at combat, either. Get Disarm Trap, Perception, Dagger, Armsmaster, and Leather Armor. You could also have your Thief handle Merchant or Identify Item if you think that your casters won't have enough skill points to spare.

Which could well be the case, since you've pretty much just got one caster each for Self and Elemental. I would recommend taking the Cleric up to Expert in Mace, Shield, and Chain Armor at some point, but the priority should be getting to Expert Spirit (for Heroism and full-party Bless) and then Master Body (for Protection from Magic, which blocks most negative statuses, and a potent Regeneration.) Mind Magic isn't as good, but it's nice to be able to cure insanity and suchlike. For your Sorceror, I would start by shooting for for Expert Fire (you're not a real wizard unless you have Fireball) and Expert Alchemy (enables "layered" potions, so you can duplicate most of the good stuff you find as treasure). After those are out of the way, go for Master in Water (Enchant Item, Town Portal) and Air (Fly). If you find yourself doing a lot of physical combat with your Sorc, get Expert Dagger too, but it's hard to justify taking skill points away from all those tantalizing magic schools.

You can also substitute for some skills by using hirelings, of course. In manyt cases, it's helpful to have some experience with the skill anyway; the +4 to Merchant from a Trader, for example, gives no benefit to characters with no ranks in Merchant and an enhanced benefit to characters with Expert or Master. But a Scholar can automatically identify all items, meaning your party can skip Identify Item entirely.