Yamcha looks awesome, cool model dude.
I'm guessing the biggest problem with adding Yamcha/Tien is creating all the animations they'd need, right?
Whatever happened with Project Tien? Did that ever get completed?
And is it possible to rig the model exactly the same as say Goku so you could use his animations? Maybe just skin the Goku model to look like Yamcha? I've used Milkshape, but not modeling software, so I'm unsure whether it'd work.
Then you could take Tien's partially completed setup and finish it. All you'd have to do after that is make the buddy system for Krillin, Tien, and Yamcha, and balance it out.
If you wanted more updated and professionally done models/animations, those could be basic placeholders to test with, so when the real ones were completed, you could swap them out and it'd be done.
EDIT: Sorry for drawing focus from your model. Keep us updated! ^_^