Until you have the 75g I would decide what kind of setup you want: whether its just a small community fish(mollys, other livebearers, tetras, corys), or you want south american (SA) cichlids mixed with community(angelfish, rams, apistos. mixed with swordtails, mollies, tetras,ect.) or you could even do an african cichlid tank(im not an african fan so i dont advocate to much for that
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the powder blue is actually a dwarf gourami and will get around 4" max, he should be moved to a larger tank though with the mollies being in the 10g. You could keep him in the 10g though with a small tetra school and some shrmp and he would be fine. treat dwarf gouramis kind of like a betta though-min of 10g though. no other gouramis and no fish who threatens them or tears ther fins, with bad breeding over years their become more sensitive to water conditions.
Also, watch craigslist. Or you can post on craigslist in pet secton wanting to know if there are any local breeders, or a local fish club. Most fish clubs will have auctions 1-2 times a year and you could get some great fish from them for cheaper than at pet stores and their normally home bred so are a bit more acclimated to the local water and hardier.
feel free to ask questions about stocking ideas, or you can google '75gallon stocking' and it will come up with some ideas, or you can play with
AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor which is a great site for different things and educatonal
p.s go with the 75g not a 55g...youll thank me later.