I have a 300 gallon stock tank with 10 wakin goldies, assorted platies, and some endlers that somehow got in there. As for tank mates, personally I’d stick with smaller livebearers. The Goldie’s can pick off fry or even give chase for some mental stimulation. Now as for a 55, I would be a bit cautious because it is quite narrow. With that said, if I absolutely had to, I’d put 2 in there but it’s pushing it. They get large and the filtration requirement can get intense if you skip a water change or 2. Good luck, school aquariums are amazing.