I have two beautiful Silver Dollars and they are as happy as can be in my 200 liter tank. A lot of people mistake these fish for red belly piranhas. Although they do look very much alike, they are vegetarians eating mostly plant matter and a tiny bit of meat.
I recently purchased three medium sized (5 cm) Silver Dollars for tank. The primary fish they share space with are four large (20 cm) Tinfoil Barbs. So far, the Silver Dollars keep their distance from the Barbs, but they coexist nicely. Two Black Tetras in the tank seem to view the Silver Dollars as competitors, challenging them in their typical territorial way, but the Silver Dollars appear to be unbothered thus far. The Silver Dollars have shown interest in tropical flakes, frozen Brine Shrimp, and dried Plankton...they are vigorous eaters. I hope they will become a bit less timid with the Barbs as they grow.
I have one silver dollar that I have had for maybe 4-5 months and he is one of my favorite fish other than my green severum. He is about 8 cm long and is in a 200 liter tank with 2 sailfin mollies, 1 gold gourami, 1 red flame gourami, 1 albino rainbow shark, 1 pleco, 1 catfish, 1 irridescent shark, 1 fat finned catfish, and 4 black neons. He is a very timid fish, but is getting more friendly every day. He doesn't bother any other fish. They say their supposedly herbivores, but mine doesn't touch plants. When it's feeding time he always snags an algae tablet I throw down for our pleco. I would highly recommend for every community tank to have one of these adorable fish.
Silver Dollars are one of the best fish you can get. Great if you want a discus type fish but can't afford it. I think a tank with Dollars, Neons and Discus would be wonderful, though.
About a month ago I introduced 2 Silver Dollars and 2 Tinfoil Barbs about 8 cm in size (all 4) into my 300 L tank which already had 2 Clown Loaches, 2 Red-Fin Sharks and a pleco. They all settled down perfectly, except for a bit of chasing by a Red-Fin Shark aimed more at the Silver Dollars, which seems to have stopped now.
We have 4 Silver Dollars in a 280 liter tank along with two 30 cm Lima Cats, a large Pleco, a 35 cm Tire Track Eel, an 18 cm Jack Dempsey and a HUGE apple snail. So far this has made for a very peaceful tank. I only comment because these four fish seem to be the opposite of the timid creatures described in other comments. Ours are the first to the top of the tank at feeding time. They compete with the JD and Tire Track to eat from our hands. They eat almost anything, including most of the Anacharis that was originally in the tank. In the first 2 1/2 weeks we had them they about doubled in size. These are juvenile tank bred fish. At about 8 cm we are just beginning to think we can sex them. I'd recommend these beauties to anyone with a larger tank.