you can't wait until they grow bigger. you are having the problem now! if you wait all that will happen is that more fish will die. also fish release hormones into the water, which control and limit their growth. if you have too many fish they will not grow properly, and this is NOT a good thing. stunted fish are unhealthy and die prematurely. its cruel and unfair to your fish to keep them in these conditions.
goldfish should not be kept in heated tanks really. its not necessary. buy another tank for your goldfish. don't bother putting a heater in the tank unless you want them to breed and then only take temp up to 16 C but you should provide good filtration and aeration.
tyler is right your clown loaches will also grow much bigger.
and if you have male and female mollies and platys, you will end up with much larger quantities once they are healthy. if you only have males or only have female this should not be a problem. the temperature you have at the moment is great for the platys and mollies.