5 gallons isn't a lot but as long as you add 1 small platy and keep a close eye on ammonia and pH, it should be fine. For a confined space, I would recommend a female, seems to be a bit more docile and suited for a community tank. You can tell the sex by the fin at the anus. A female has one single fin, while a male has two smaller fins and a small tube-like protrusion inbetween. 1 female will get on fine and will clean algae out of the tank, while also feeding on flakes and bloodworms (if you feed those)
One platy should be fine by herself, and will still make a neat addition to the tank. The black tetra won't be putting any real strain on the tank setup and the white clouds aren't huge. Just make sure that you check the tank parameters frequently for a week or so until the fish is settled in and the tank has compensated.
Good luck