Provided you have followed my instruction to the letter, you don't need to do anything except monitor the pH. This fishless cycling method was written by a chap on another forum (I pinched it but didn't copy, if you see what I mean
). The old fishless method involved adding ammonia every time that dropped to zero, but that made so much nitrite it stalled the cycle. The critical point is 14 ppm nitrite but our testers can't measure that high and we would need very accurate measuring equipment and pure water to dilute a sample and get a meaningful result on the tester scale. So the other chap designed this new method in such a way that nitrite can never go above the critical value if it is followed strictly.
The reason I say to monitor the pH is because you have soft water. When GH is low, KH is usually also low. There are two problems with this. Nitrite and then nitrate are acidic and with low KH the pH could crash - and the filter bacteria don't like very low pH. And the bacteria need inorganic carbon (ie carbonate) to grow properly. If you find the pH is falling or even has fallen, all you need to do during cycling is add some bicarbonate of soda, sold in small tubs in the home baking section of the supermarket. Try 1 x 5ml spoonful for every 25 litres water. That's flat teaspoons as measured by a proper measuring spoon.
You can't use bicarb once you have fish, but regular weekly water changes should keep the KH topped up, or you might find you need to use some calcium containing decor (eg limestone rocks) or even crushed coral in a bag in the filter if you can fit one in.
As fcmf says, you just need to avoid fish that need hard water.
Dwarf cichlids will love your water. The easy to get ones are several species of Apistogramma and kribs. Some shops may also stock fish from the Nannacara and Laetacara genuses. (Should that be geni
)
Dwarf gouramis will be fine - though you do need to bear in mind that a lot of this species are already infected with an incurable disease by the time they get to the shop. Don't buy any fish from a tank that has iffy looking gouramis in there. Or look at honey gouramis instead.
You don't mention which rainbows, but with a tank your size you can't really get bigger than dwarf rainbowfish (Melanotaenia praecox). The lower end of their range is 5 deg GH so you might get away with them. If you like the look of them, featherfin rainbows (Iriatherina werneri) would be an option as they can go as soft as 1 deg GH.
If you want any shoaling fish, just look at the species native to the Amazon and its tributaries.