They may be small now, but silver dollars get to be over 6". So just the 4 of those would need at least 24gal of your tank. The barbs get to about half that size at 3" max so that's another 12gal of the tank. So that leaves a good 19gal of tank left to play with (getting rid of the goldfish that is). Have you thought about some bottom dwelling fish to break up the soon to be crowded upper part of the tank as your fish grow? Some
Corydoras Catfish or
Otocinclus Catfish would make nice additions. You could even get 3-4 of each and have a really nice clean up crew for the tank as the Corys eat all the left over food that drops to the bottom and the Oto's will eat any algae you get. Just something to think about to round out the tank.