Male guppies can be a real PITA, as you are finding out! They are beautiful, but when they decide to be pesky, it seems nothing will stop them. I had just male guppies and nothing else in my 20 gallon at one time, and they were bugging each other. I took out the main culprit, but the same thing happened to me as what happened to you--the remaining guppies then started bugging each other, and it seemed like one would always get the worst of the bullying. Endlers are much the same way, although not quite as persistent. I have to keep my black bar and tiger endlers apart, or the black bars will bug the tigers to no end, since there are no females in that tank. I guess they think the tigers are females. Male guppies will go for just about anything female, be it a molly, platy, swordtail, or other similar size fish. I've never seen them go after larger fish though. You might try keep only one male guppy and a Honey gourami, since both would do alright in a 20 gallon. NOT a full size gourami. Honeys are smaller--no more than 2 inches when grown. Honeys are usually very mild in nature, unlike most other gouramis, and I've yet to see a male guppy bug one. Don't get more than one though, as honeys will often fight--and a 20 gallon will make them more prone to do so. Male guppies also don't bother rams, but make sure you only have one ram if you go that route, as more than one will either mean them breeding, and ending up killing the guppy, or they'll fight. Rams are also very sensitive, and not the easiest fish to keep. You might try a few male endlers with your guppy as well, but there is no guarantee that one or two won't be picked on. I guess it depends more or less on the personality of the guppy. Some are a lot more mild than others. What is the temp of your tank? Warmer temps will make guppies more active, and therefore, more likely to bug each other. I have my tank set right at 80F because of my ram, and my endlers are super active. Endlers are charming, but a lot of people complain that their LFS
don't carry them. You might have to order online.