a good place to go for building characters is
http://www.geocities.com/jandrall
If you want a thief, replace the Fighter. If you follow the directions there for a rogue. In Wiz8, Rogues can compete with Fighters for Damage after awhile (you need good stealth score).
Or, you can use a Rogue instead of a Valkyrie. Early in the game, you can recruit Vi Domina, she's a Valkyrie. My party is Sam-Fig-Ran-Gad-Bar-Bish, and I do okay. My next time through, I'm dropping my Samurai and Trying Fighter-Rogue for my front line, I'll probably not use a gadgeteer the 2nd time, I put Bards and Gadgeteers in the same quirky "utility-man" role, but I find Bards suit my play better. I'll give the Fairy Ninja a try. Since I'll have 3 Classes that have the Locks/pickpocket skill, it will allow me to use my skill points on my Ninja towards combat.
As for your party... instead of a Mage, I'd use a Bishop instead. Going with one major magic user is fine, but if you do that, make it a Bishop... just personal preference, and most likely the preference of most people on this board.
Ranger with a Triple Crossbow is quite the critical hit dealer and there are plenty of weapons mid-late game that give criticals for a fighter.