Is this a good tank set up?

ok i just bought 2-1 inch red belly piranhas i have a much bigger tank for them but since they are only 1 inch im going to wait until i put them in the 55 gallon tank..... right now they are in a 20 gallon tank which includes a 1 inch convict cichlid and a pleco.... so is this good enough for now? also i have been told that 2 piranhas arent enough and i need at least two more is this true? thanks for any help