Good question... course... BB doesn't even need to become G-Rex...
Beast could probably take on any one of BB's transformations, but BB isn't going to try to stand and fight as a gorilla. He's not stupid. He's a succesful hero, he knows how to use his powers. And he may tire, but he won't start sweating rihgt away. One transformation is like nothing, so's the next. The strain increases with every morph, and he's pretty damn tuff.