It's been years since I've had a "real" freshwater aquarium (right now I have a 3 gal that's home to Kyle, my betta -- my other tank is saltwater). I never had problems with my pleco and my goldfish; I did, however, have problems with my goldfish and ANY other tropical species -- the goldfish were aggressive towards them.

It's hard to resist the temptation to add "just one more fish" but, as mentioned, bioload is of concern. Is your tank planted? If not, how about adding some live plants? They can add decoration and be beneficial as well.

We'd love to see pics of your tank if you have any!