Toffee: YOu are right about the fish being found in rice patties. The problem is, the rice patties, although shallow, are big. Think of a five inch deep aquarium that is an acre square. so, even though they don't have a lot of verticle space, they do have a lot of horizontal space. A bowl doesn't have a lot of vertical or horizontal space.
You are also correct in them being found in mud puttles. But, that is just an example of how well these fish survive in poor conditions. They do not live in mudd puddles all year long, or for any extent of time for that matter.
To answer your question:
No, you shouldn't add another fish. Being anatobids (sp?) paradise fish do not get along with each other well. Even if you have a male and a female, they wont like being together in such a small space until they are ready to breed. Think of them as bachelors, they don't want to be in the same house as a women unless they're having sex. (ok a little exageration)
If you want to keep your fish in a bowl for now, and are succesfull in doing so, then go ahead, none of us can stop you. But, just because you are doing this succesfully does not meen it is the best conditions for your fish. Rainbow trout can be raised in a tube with a lot of other trout, packed like sardines, but that does not meen that that is the best environment for them.