If possible, I would recommend a 3-4 ft tank such as a Fluval Roma 240 for the following reasons:
1. larger aquariums tend to have more stable water parameters
2. allow for a much larger range of stock
3. external filters are easier to look after
4. look more impressive
5. unlikely to ever require an upgrade in size
Then, I would strongly recommend a fish-less cycle over a fish-in. Seeding the filter with established media will speed it up very often.
Finally, most fish do much better in larger schools, so for an aquarium that size (fully fish-less cycled), I would be looking at something like..
* 25-30 mid-water schooling fish of one species
* pair of 4-6" cichlids such as Bolivian rams or keyholes
* harem of dwarf cichlids such as Apistogramma cacatuoides
* 10 Corys of one species or 1m 2f plecos under 6" (for example bristlenoses) or 8-10 dwarf chain loaches or 6-8 zebra loaches
If you are really determined to get as many species as possible, something more along the lines of..
* 15 mid-water schoolers
* 10 small hatchetfish
* 1 4-6" gourami
* pair of medium cichlids
* 1m 2f or pair of small cichlids
* 8 Corys
* 6 Otos
* 1 pleco
..but I really strongly recommend against as many species are possible. I also feel that their behavior won't be as interesting then and the tank won't look as good.
Exact stocking does, of course, depend on your water parameters.
