Silver dollars do eat plants, so they may ruin some, and they like to be in shoals of 6 or more for the feeling of saftey. For 6 in a 50 gallon should be ok, but I don't think you could put any more fish inside it.
Silver dollars do alright in a 50gallon, but I'm planning to move mine to a 100 gallon in the immediate future, and they're nowhere near full grown yet. They're beautiful fish and wonderful shoalers, but they're extremely nervous and shy if kept in small groups; they require their own kind to feel safe. They are very herbivorous, but I manage to keep mine in a densely planted tank. If I get negligent about adding fresh veggies, however, they start to mow down my plants with godspeed.