There is some great maturity all levels stay below 10ppm.
Ammonia and nitrite should be at zero, completely undetectable, in a "mature" tank. 10ppm Nitrate could either be from a well maintained tank [water changes] or an unestablished filter not able to convert all the waste from ammonia to nitrate. Detectable ammonia readings coupled with low nitrate is a good indication that there isn't yet enough bacteria to handle the waste from your fish. With the way you've set it up you should be well on the way to having a well established tank [IMO you can't "buy" maturity with bottled products] so just keep up the regular water changes and try keep the ammonia and nitrite as close to zero as possible. (and forgive me if by "all under 10ppm" you were meaning 0, 0, and <10).
It's a great looking tank for a first effort too! I think it is big enough for the three oscars, though I'd look at rehoming the ray before it gets too big [or dies], there's plenty of other oscar-proof bottom dwellers that would do alright in there.