wanting to start a Q tank

I currently have a 50 gal and have been having problems with my oscar in there. I really want to get a Q tank set up now and was just wondering what size would be best for my current and any future fish. Obviously I do not wanna get a small tank if I am keeping oscars and the such due to the size of the fish (mine is currently about 4")

For future though would a 30 or something be better to have in case I want to get bigger fish added to my current 50 or would it actually be ok to do a small 10 gal for a few weeks until the fish get a clean bill of health to be added into the big tank?

got a rhino pleco and a jewel cichlid in there with it now. The jewel was some chick at works her husband bought two of them threw em in a 30 gal community tank then wondered where all the other fish went. She kept them for a little bit feeding them pellets then sold everything to a guy at work (fish tank stand filtration heaters decorations EVERYTHING) he is a good friend of mine and had both jewels for a few months then one of them died he says he never could get them to eat anything so he started them on feeders, now I am trying to get it back onto pellets. The friend got sick of the single jewel because it mainly just hung out inside a cave so instead of letting him flush it I had just recently torn down and set my 50 back up and took the jewel went and bought the oscar and rhino and all three have been doing fine ever since (outside of the health issues with the oscar)

Oscar ate the jewel a few weeks ago so it is just him and the rhino which does nothing but poo and sleep under the driftwood until we all go to bed.