I heard for the second movie that Rocksteady and Beebop were going to be used for the mutants, but the creaters of the turtles comics did not want to use them as they felt the henchmen in the cartoon series was bad enough.
The second one was alot less violent than the first one, where the first one had the turtles using their weapons and using colorful language. The second one had the turtles not say any fowl language and they did not use their weapons that muich if any. Look at Michaelangelo, he did not even use his nunchucks once in the movie.