If you have a medium sized bowl and have a Siamese fighting fish in it is it okay to keep another fish in it or would it be too full?

There are not many fish that can be kept with Siamese fighting fish. They are able to survive in small bowls because they have a breathing organ (a labyrinth) that allows them to breathe some atmospheric air. While they will get on with most catfish, livebearers and other small fish, these other fish will not be happy in a bowl. If you add a filter, you could keep some pygmy corydoras, otocinclus catfish, ember tetras or white cloud mountain minnows. Each of these are schooling species, buy at least 3 and be aware you will now have a lot more water changes to do.