It's unfortunate that bio-load is always the only thing considered regarding aquarium size.
Yes, you can keep 2 paradise fish in the 5 gallon as long as you do enough water changes to support the bioload - this is not why I wouldn't recommend them.
They can be aggressive toward each other. The smaller the tank, the more aggression will come out. These are also fish that, kept properly and allowed to live long enough, will reach 4", sometimes a little larger. Fish don't need just clean water, they need space to live.
I don't believe there are many fish suitable to such a small tank, but it also depends on how much the lives of the fish are valued by the owner. I think the betta is as nice a suggestion and you can get on this one, and you can make a charming microtank with it.