I've noticed it's pretty common for a bigger Kisser to get aggressive with a smaller one, especially if they are both males. You might want to seperate these 2. It can get pretty bad, and you don't want the little guy getting hurt.
Sometimes what can help, is adding more plants and rocks, and caves and such, so they can establish their own territories. I personally have had to keep most of my Kissers seperate from each other, because of their aggression. They are all fine to get along with other fish though..including little fish, like Neons and such. Strange Fish.
Also, you're aware Kissers get pretty big..8-9", right?