Remember they are getting a goldfish. They are cold water fish and won’t do well with a heater.Plecos are very hit and miss when kept together, it's true. There isn't a lot of info available with regards to the aggression levels of Dwarf Plecos to be able to research beforehand (unlike Cichlids), but I currently have 1 Goldseam Pleco, 1 Leopard Frog Pleco and 3 Clown Plecos that live all happily in a 60G. I really just took the gamble and it worked out. Back to the original question, I would get a Jaguar Catfish and a heater. They are the No 1 Catfish money can buy, not too big, not too aggressive and so cool and nicely marked.
Goldfish aren't coldwater fish. This is a misconception that many people believe. Goldfish can handle warm waters just as much as any other tropical fish. They just have a higher tolerance to colder water.Remember they are getting a goldfish. They are cold water fish and won’t do well with a heater.