As I see it......There are at least two problems to consider here, the long term and the short, depending on what your goal is.
One might be...Do you want to keep the oscar in the same tank with the Thorichthys? Will they ever accept it as a tank mate? and are you thinking at 6 ", the oscar will surpass the growth of the FMs and be able to hold its own, and coexist? Orat that point will the oscar seek revenge (I know, way to anthropomorphic)
Or are you buying enough time with the 20 gal to acquire an adequate size tank for the oscar alone, a 125?
Whether the oscar will last in a 20 gal is all about water changes.
At 2", a normal water change routine will be adequate (50% per week), but for each inch it grows, you will probably need to double your amount of changes, and frequency. (although in a tank as small as a 20 this is not that big a deal.
By 6" I would guess to keep the oscar healthy, you will need to do "daily" 80% water changes.
The other consideration is whether or not the FMs will ever be able to share a tank with the oscar.
I'm guessing the FMs are established residents, and own the tank??
So if the entire tank is their territory, will they ever accept the new guy?
50/50 yes, 50/50 no, and since they have extended ownership, they will always have the most to lose by allowing another cichlid in,
so probably fight to the death to keep it. And if the oscar grows large enough, this may be the case.
I normally do not mix South American and Central American cichlids in the same tank.
Besides the differing water parameter requirements, the normal aggression state of Central Americans is much more heightened than their S American cousins from east of the Andes.
Even though I consider FMs, on the mild mannered side compared to most other Central Americans, their territoriality is much stronger than any oscar's normally passive nature.
