Yeah, you're right. The Wiki says it was Rudman and Robinson performing Sweetums here.
It may just be that, since Scooter wasn't in the opening, David Rudman was able to perform the character. Scooter's in the ending, so maybe he wanted to focus on performing that character. Or Marty may not have been available for the opening.
I don't recall Sweetums saying much of anything at the end. Remember, performers are always switching around like this when a character may have very little to do in a scene.