I've got 19 in a 75g with guppies, endlers, honey gouramis, L387 and bristlenose plecs, and kribs. I've run into absolutely zero fin nipping. They are fairly loose schoolers at the moment, but that might change if I added a larger fish. I heard that most schoolers will tighten up the school if a larger fish is added. At the moment, my diamonds are the largest fish in the tank, except maybe for the male kribs that are about the same size.
I find they stay in the middle, but may stray into the bottom or top at times. I don't think they are hard to keep at all. I'm not the most consistent with my water changes, but I try to do 50% weekly. As always, ammonia and nitrite are 0 but nitrates are usually over 25 (tap already has 10-20 depending on the season). They attack any foods I drop in. Even practically jumping out of the water to grab the food.
What else? Umm, they are super gorgeous! I've had mine for several months, I think 5 months. They just keep getting prettier and prettier. When I got them the largest was about 1.5". Now I think the larges is about 2". The fins, especially on the ones I think are males, get so dramatic. They are turning a sort of lavender color and are really the dorsal is so long and flowing.