I have 30" l X x 18" w X 18" h tank with 21 goldfish sizes from 1" to 2", 2 mollies & 1 male betta. I want to add some other fishes with them, pls tell me which fish i should add with them.

that seems like you have more than enough fish in there. goldfish get really big and shit a lot which means you'll need high filtration. they will outgrow that tank i can guarantee that. i would take some back or give them away. you could also buy an oscar for that tank. it'll take care of your goldfish problem.

You asked and you're not going to read the answers you were looking for from anyone, I'm afraid.
As its already been stated, your tank is overstocked.

Secondly, the fish you already have aren't compatible. Mollys and bettas require warmer water temperatures than goldfish for longevity. For example: the optimum temperature for goldfish is 68-74, while your betta and mollies want something closer to 77-82. It doesn't seem like much but it can make a large difference over a long period of time.

Third, MALE bettas aren't really recommended for most community setups or even the mixed setup you have.

How long have you been running this tank like this?

ya ill agree with the goldfish shit alot.... we have a 400 gallon and a 1000 gallon pond and twice a year i have to drain it and shovel out LITERALLY a combined probably 150 lbs of welll shit haha