You should probably keep no more than two of them, unless you want to start a bachelor pen. They'll begin to fight over the hens if there aren't enough hens for the roos, and they'll really over-mate the hens.
Of course, you could just get yourself up to 30 hens, and then you could keep them...
I agree, Grumpy will only get worse as he matures. Give the Professor a chance at the top spot.
There are too many extra (sweet) roosters in the world to keep an aggressive rooster.