Gold sevs are cool fish they grow to be quite large , I had two that got to be around 8 inches. They are pretty peacful until they mate. Will eat your danios, betta ,and swords for sure. May get your groumis too. especialy when mating. They will even bite you while mating.
Hopefully the 55 will be for them only. You can add some other fish too . I had mine with silver dollars. and large angel fish. Did quite well toghether.