Water temp - 86 degrees F.
pH - anything stable above 7, I would say... a lot of experienced keepers recommend 7.5 - 7.8.. Mine's stable at 8.2.. don't wanna mess with my tap water pH every water change.
20's a lot.. the 3 inchers will be the dominant ones.. I had just 6 in a 55 gallon and I got rid of 5 of 'em in like a week.. they were already fighting with eachother.. but mainly because I wanted my keeper to have more room to growout..
I think if you have 'em in there without any decorations.. or anything they can make territory, then you should be fine for a little while.. until 3 - 4 inches at least.. ie., flat, even gravel(no bumps, hills or no gravel at all), no plants, no decorations, etc.