You should have no problem with the Buenos Aires Tetra. Species is Hyphessobrycon anisitsi. One of the hardiest aquarium fish but it has behaviours that must be considered; it is very boisterous, can be a fin nipper, and will usually eat plants. When kept in small groups (less than 8) it can be very nasty. It prefers the middle reaches in the aquarium. An albino strain has been developed. Males are more colourful, particularly in the unpaired red fins, and females are rounder. A typical egg-scatter that is easily spawned; parents will eat the eggs if not removed immediately after spawning. I would get a few more to have 8 in total. This fish should attain close to 3 inches (8 cm) but can reach 3.5 in (9 cm) and is very active.
Corydoras catfish, a group of 15-20, one species or a combo, but if the latter it is preferable to try and have a few of each species. I tend to aim for five, but often end up with three for whatever reason, but it does seem to be the number in its entirety that does matter most.
This would fill the tank, but you could also consider maybe a common whiptail (one, two or three) of the species Rineloricaria parva which is the common and smaller "whiptail." Upper level fish, close to the surface, is not easy here given the traits of the BA but a group (7-8) of Penguin Tetras (Thayeria boehlkei is the more common species) might be OK. Hatchetfish which I would usually suggest for the surface would almost certainly be nipped to death.
I like this tank size; I have one that I acquired four years ago when I was downsizing (got rid of my large 4 and 5-foot tanks) preparatory to moving, and it is a very serviceable tank. Photo below is what it looks like now (not a very good likeness due to my aging cheap digital camera); my group of 41 cories and a couple of upper pencilfish species that are living out the end of their days before I decide on new uppper level fish.
