I think...
Kat Von D over Bullock - Kat seems tougher, would fight dirtier. Bullock would get stomped. Kat would probably wind up knocking Bullock out.
Jolie over Aniston - similar reasons to the first contest. Jolie is tougher, and crazier. Jen submits to Jolie.
Hatcher over Cross - Hatcher quite easily over Cross, in fact.
Avril over Lindsay - Tough contest, with Lohan leading for most of it. Lohan gets cocky and Avril surprises her with a big finisher to take the match.
Mendes over Simpson - Exactly what Mac4 said.
Avril over Hatcher.
Cox over Hatcher.
My fantasy fight would involve Sophia Bush. Maybe against Hilary Duff or Emmanuelle Chriqui, or even Jennifer Love Hewitt? With lots of taunting and breast attacks. If anyone ever wrote something like that, I'd feel like a kid in a candy store.