To me, you seem pretty fully stocked as it is. The plecos, assuming the are the common plecos, will outgrow your tank and you must have an extremely strong filter for them because of how much waste they create. You'll have to rehome them. You could probably fit a nice gourami i there once the plecos are rehomed, maybe a nice opaline gourami, that would be cool. Your beta might become aggressive at some point, assuming that it is actually a male. Oh, and before you add any other fish, please up the catfish school to 6 catfish, because I think that you have cory's and they do best in schools of 6.