Well let's not jump the gun here. Rossi is a great rider certainly, but if anybody thinks he is going to waltz into F1 and dominate just because "he can win on two wheels so four shouldn't be a problem" they are dreaming. Lots of cross platform jumps have failed, in fact, who really has truly dominated in both equally? Even Mike Hailwood got raped when he tried to transition his two wheel legendary status to four wheels.
What's most likely to happen is Rossi will jump into the fray, become a contender, get spanked by Schumi and slapped around by some other guys, then realize as a driver he is 2nd tier at best or behind all the 1st tier drivers. Winning in one format doesn't guarantee wins in another. Even among 4 wheel formats, it's very difficult going over to something radically different, Andretti being the only obvious exception.
My two cents. That is all.