In my opinion, you already have too many fish in the tank. Silver dollars grow to six inches, and a 20g is not sufficient space for such fish. And an Oscar
attains 15 inches. I'll explain why this is not recommended even when the fish are young (small).
Keeping a potentially large fish in a small tank negatively affects its growth. And unlike humans, most fish continue to "grow" throughout their lives. But, this is not a benign process that results in a miniature version of the adult just because it is in a small space. What occurs is termed "stunting" which involves numerous health problems as the fish develops (grows). A defective immune system is one common result of stunting, which results in the fish contracting diseases and issues that it would otherwise be able to fight off. In addition to the physical size limitation of the small tank which causes considerable stress to the fish, there is a problem with the water chemistry in a small space; the fish is literally being slowly poisoned by not having sufficient water.
Young fish must be transferred regularly to larger and larger aquaria as they grow to allow for normal development. Unless you are prepared to provide adequate housing throughout the fish's expected lifespan, you should not consider acquiring such fish.
The silver dollars are already in this predicament, as their adult size of six inches, assuming they are able to live long enough, will require them to be in a much larger tank if they are to be healthy. And they are shoaling fish and should be kept in groups, minimum 5 or 6 but there is insufficient space in a 20g for even two. If larger aquaria are not possible, I would suggest trading the silver dollars for smaller fish that will be good companions for gouramis. The Rainbow Shark
also attains six inches, and should be re-homed.