Yep, there's all sorts of fish out there. Rasboras and dwarf cichlids are bred in 5.5 gallon tanks by folks on these forums, whereas a 180 gallon tanks are generally what oscar breeders suggest. The one thing I would keep in mind is widths of the tanks you choose for building your racks. For smaller tanks like I'm guessing you mean, they're usually either 13" or 18". And 18" are usually the tanks you see labeled as "breeder" tanks, ie. 40 breeder, etc. Also, while it's not ideal for viewing, you can fit a lot more tanks on a rack turning them sideways. That would drive me crazy personally, but if space is really limited it can help save a ton.
Nothing good happens fast in an ecosystem.