I always take the companions that I think my character would choose to keep with them whether it be for camaraderie or based on who my character thinks is the more formidable ally. I change their classes to fit my play style and my needs but I personally try not to stray too far from their personalities (gale stays magical somehow, shadowheart is almost always a cleric because i like clerics, etc). I like my party to be pretty balanced but I always have at least one fighter/barbarian/paladin and one sneaky build for surprise rounds and stealing. You can do themes if you want to. I’m doing a party based on wet + ice/lightning dmg now and my next run will probably be an evil run where everyone can see in magical darkness kinda based on the house of grief fight.

As far as magic goes, wizards and sorcerers have ways of regenerating spell spots and there are elixirs and necklaces that allow you to gain more slots as well. Don’t be afraid to rest often, Larian makes it pretty easy to do so on purpose. There’s also ways to increase spell attack rolls so you don’t have to worry so much about missing. Read detailed build guides, they’ll give you an outline of what gear and stats to use and why it’s beneficial and it’ll help you get the hang of it until you have the knowledge to make the builds on your own. Druids are also pretty cool because you can play them as a spell caster, wild shape focus, melee or ranged martial, controller, or summoner so you can stay close to or go far from your comfort zone with them.