Go to your LFS, have a look arround, and make a list of the fish you like, then report back. We can help from there
What type of pleco is it? You should bear in mind that many plecos will grow to over a foot long It may be an idea the start thinking about what you need to do with him when he outgrows your tank
In a tank that size you have a hell of a lot of options, you still have a lot of space to play with depending on the species of your plec. Can you post us a photo to ID him from? I would recommend things like: