Would there be serious flaws in a party of

A sorcerer
Dwarven fighter/cleric
A paladin
A thief of some sort
And maybe a mage/cleric?

Any suggestions? What kind of characters make good thieves and is some kit especially fun to play?

Well, my opinion is that there can be no serious flaws in any party of any kind... Remember, people are soloing every class, right? That looks like a fine party, though. As for a thief, what type are you looking for? Do you want a backstabbing powerhouse that could possibly step into melee when needed, or do you just want someone to detect traps and pick locks? Are you using NPCs at all?

I don't think I need another melee character, just someone to open stubborn doors and untrap traps. And maybe use a bow.

I might temporarily pick up NPCs to solve their quests. But I don't think I'll be keeping them.

If you want them to use a bow, I would recomend downloading "Song and Silence"

It adds a "Sharpshooter" thief kit. Very useful.

I think I have that mod. The kit seems pretty nice, I might very well use that one. Thank you.