My experience with green chromises (chromi?) was that if you are lucky enough to get a good sex ratio, you won't have as many problems. I started with 5 juveniles, 2 turned out to be male. The two males fought constantly, but things were relatively ok between the females. If I had gotten more males, I'm sure there would have been deaths.

I do agree that 20 gallons is a little small for more than 1. They get quite a bit bigger than you ever see them in stores, and if you got two, and both turned out to be male, it could be ugly.