I have two sail fin plecos and 3 chinese algae eaters that are in my pond with my comet/common
goldfish. I had to bring them in for the winter since my pond is not heated, but they do a real good job at keeping the algae cleaned up in my pond. Some people have issues with
CAE's being aggressive towards the other fish in the tank, but I have not had any problems with mine. Since you have a smaller tank than my pond, you may want to consider one of the smaller plecos that can handle cooler water temps. The main issue is going to be water temps since goldies need cooler water temps. A weather loach may be ideal since they can take the cooler water. I haven't seen any around here, so I don't really know that much about them.