The scene from Teen Titans was a wink/nod to when BB first turned into an alien-animal back in the 80's, during the "Who is Wonder Girl?" arc in The New Teen Titans (Vol.2), issues 50-54 (inc. 55 where she first became Troia); so, he's actually been able to turn into alien/otherworldly animals for pretty much the long haul of the character. However, even despite is greater power/finesse now, I don't think he could turn into Krypto (as that would likely require more raw power than he possesses, inherently), and I don't think he can readily defeat Colossus, except perhaps through some weird BFR. Conversly, however, if BB stays smart/sharp, I don't see C beating him, either, as long as he stays mobile and doesn't let C get too many direct hits.
So I guess I'd say it a general wash, with C taking it if he can land some solid blows, and BB taking it via BFR, or similar.