You should be able to keep 3-4 Japanese Fire-bellied newts in a 20g. no problem. I've never kept mine with fish so i wouldnt know what would be compatable. Just a shot in the dark but i would recommend a higher level swimming fish that either is larger or a lot faster than the newts. I guess zebra danios would work but may get eaten (once again just guessing). In terms of breeding them obviously your going to need to find a male and a female. The easiest way to tell them apart is that the male will be a little swollen at the underside base of his tail during the breeding season. First though they have to hibernate. This is more easily accomplished during the winter or cooler months. If you live in an area that has about a 6-8 week stretch of 46(or so)degree temperature place the newts in a plastic container filled with water and some gravel and have someway to keep the container inside dark. If you're willing to give up some refrigerator space you can have your newts hibernate during the summer and it would be easier to regulate the temp. After the hibernation period return them to the tank. The tank temperature should be around 68 degrees. Once the female has laid the eggs remove her or the eggs since she may eat them. If you decide to remove the eggs just put them in a similar set up with similar water conditions. Once the eggs hatch just feed the larvae daphnia, cyclops, or chopped bloodworms, tubifex worms, or whiteworms. Also try to find some books or more info on the topic, the more info the better. Good luck and tell us how it goes.