I would not bother with ammonia. I never do this, as I always have floating plants. I expect you will want these, they are natural to all tropical forest habitats (either actual floating plants, or overhanging marginal vegetation or forest canopy which achieves the same effect of dimming light and is frankly necessary for the fish). Plants take up ammonia, and quite a lot, from fish and decomposition.
I did have a Tetra pH kit that went down to 5.0, but it expired and I don't know if they still make them. My API has 6.0 as the lowest, and several of may tanks are much deeper yellow that this so I assume they are down at 5 or less. Even when I had the Tetra kit, some tanks had deeper yellow than the 5.0 colour.
Wood is usually not much to create true tannic brown-tinted water. Dried leaves do this much more. My blackwater tanks do not have tinted water, or more accurately, it is tinted slightly and I really don't notice. My 10g that I use for the pygmy cories and growing out Farlowella fry does turn noticeably amber depending upon how many oak leaves I put in it. For the last batch of fry, I had so many oak leaves thee water in the bucket was so dark you could not see the bottom, and that was a off-white bucket. My 40g is Amazon blackwater, here's a photo of it as of last week. As I said, it needs a renovation, as the pygmy chain sword has spread everywhere and most of the branches have fallen apart.
I had a trio (all that were left in the store) with some galaxy rasbora a few years ago in a similar sized tanks and they did pretty well, didnt get very long but did get quite fat! Funny little fish to watch a group would be good, you may be able to find them but its always a bit tricky with some fish, Im sure certain fish get imported from different countries at different times so it may be worth keeping a look out. 
