Can a collection agency still sue you, if you are making monthly payments; even though its not the amount they want you to pay?

I really don't think so. They'll just bill you and bill you. The damage that is done is your credit. It took about two years to pay off a bill that was in collections. They never threatened to sue... as long as I sent them money every month.
Oh... on the other hand... I know someone that hasn't sent one payment for their collection which has been about 7 years already, and they just keep sending the bill. So I guess it's really up to you and how you want your credit score to be. That would be the two sides to it.