There have been some threads on spells and I'd say that there is no real concensus on what are the best spells to have. Players seem to have different favorites. The spell Blood to Gold is universally hated (justly so).
I'm not sure I understand what you mean by mixing spells together - you can cast multiple protection spells on your characters, or spells like Bloodlust which can be very useful for the party.
Learning all the different types of spells is a matter of changing from role to role.
Do a search on spells and you should find some more input!
Wyv