So you bought the tank from him as well? Because you say they're in the same tank. Or do you mean they were also in a 55G which is what I imagine you have them in as you also don't answer which tank they're in. You list your tanks so I'm assuming they're in your 55G-"Semi aggressive tank". And how do you know your water quality is better? The only parameter you mention is ammonia. What are your other parameters? Did you test his water to be able to tell yours is "better quality"?
You use common names so again I have to guess about some of your fish. The Black Bumblebee I'm guessing are Tilapia buttikoferi? Or is it Pseudotropheus crabro? Or a Bumble Bee Oscar? I also don't know your full stock because you list 12 fish at first then throw in the Bichir at the end making for 13 fish in a 55. Are there other tankmates you haven't mentioned?
Even if it's 12 that's a lot of fish for a 55 especially considering your stock. If any of the Bumblee cichlids are the species I mentioned a 55 isn't going to be big enough for 8 of them for very long if is even now considering how aggressive some of the species are. That combined with Convicts which are also notorious for being aggressive and I can see why you're having issues. The Gourami's have long filament pelvic fins which are often targeted by many fish especially cichlids so that explains the missing streamer and as I stated before the cichlids you have them housed with are fairly aggressive which explains the missing fin pieces. Aggression even if the Gourami's are moved would sill exist amongst the cichlids and your birchir would likely become a target as well.
Temporarily housing with the Goldfish as
SalviniCichlidFan
said isn't a good idea. While possible it would require you to raise your Goldfish tank temp which can be very problematic for the Goldfish as they live in cooler water because of their need for more dissolved oxygen in their water. Also Goldfish produce copious amounts of waste which could affect the Gourami's and the Gourami's may pick on the Goldfish. Just to name a few issues that may arise. Unfortunately, your best bet would probably be to rehome your fish and research more on the fish you'd like to add to your 55 for the near future.