Finally started my playthrough, is this a decent party comp? Half-Giant(Cloudtop) Fighter, Human(Half-Elf) Mage-Knight, Human(Argossian) Champion, Elf Wizard, Human(Argossian) Druid, Human(Southerner) Bard. Only in Finchbury so far, but have had no problems. Is the lack of a psion gonna hurt me?
What difficulty. Yes, it matters for the answer. If on "Challenging" (normal) then you might be ok. Might being the operative word.
If on "Archmage" (very hard) then very probably not unless you are a genius of d&d tactics, and i don't mean it like one of steam reviewers "i have the manual for 3.5 memorised". You have only 2 pure casters and 3 hybrids. That's very few and too much respectively.
As a minor issue, you have 4 humans out of 6 characters. This isn't good, there are occasions where racial skills play a role, can give additional loot or prevent ambushes. There are at least 3 good races+subraces for each profession to choose from. But this is minor, want humans, play humans.
I am still relatively early in game so take this advice with a grain of salt. I would consider:
Adding a Psionicist - they are great and how they cast "spells" is versatile. They also have have early game more spells points equivalents to spell slots.
If not then a Cleric or another Wizard - of different colour for a change.
Adding a second pure melee. I have a gladiator but people says that Samurais and Death Knights are good too.
Fighter, Samurai, Psionicist, Druid, Wizard, Bard for example.
But as i said, on normal you "might" be ok and my knowledge is limited anyway.