This should not be applied in general. Most are, some aren't.i would not do tetras. they are too nippy and active.
You were talking about suggesting them in a a tank larger than 15 gallons. His tank is only 5 gallons therefore I would not suggest anything else more than the betta there.my female bettas have been great with a school of 5 hatchetfish, though. on the other hand, they couldnt stand the activity of 3 zebra danios. corys would be great, as would otos. any sort of non aggressive bottom feaders and algae eaters are ideal. bettas can be perfectly content living alone - some individual bettas have issues with certain species that others dont. experiment!
A female betta is not advisable to be kept together with a male in the same tank. Chances are the male will bully her to death.Maybe a female Betta and a Otto cat