A 10G is a terrible tank to keep silver dollars in, sorry, but in my opinion you should not have bought those fish. I have read people who have had them go from 1" to 2.5" in a couple of months, even at only 2.5" they are going to be very cramped in a 10G, let alone when they start getting into the 4"+ size range.
Your 20G also sounds pretty crowded with 3 large angels, I have 2 in a 55G and they use up every inch of space. If you end up with a pair out of those 3, what do you plan to do with the odd one out?
I realize $1 is a good deal for any fish, but just because the price of the fish is good on your wallet does not make them fit any better in a small tank.