yes it is dangerous for them to be in that small of a tank. and yes it is detrimental to their health. They will get LOTS of different illnesses, and keeping their water clean enough will be IMPOSSIBLE!
They do NOT grow to their environment, they grow until they are full grown. IF you put an Oscar in that small of a tank, it will have a miserable slow death. and it will be torture.
There are a lot of here who spend an unbelievable amount of time, learning everything we can about HAPPY fish keeping, and teaching others. Trust me when I say, NO SMALLER than 55 gallons.