90G is a lovely size tank but you are really wasting it having like 3 tetra, 1 kuhli loach.. You should get another 5 khulis, so that they are active. I think another 12-17 cardinals would do best, big groups look stunning and behave much better. If you like guppies you should also get more of them as it will feel intimidated by the larger fish on its own.
Your Tiger Oscar will be a thread as soon as he starts to mature, usually about 4" +.
I would forget about adding any new types of fish, increase the groups of what you have already, and be prepared for that oscar to grow substantially and start eating your fish unless you plan to move some of them out. Oscars grow quick, and they need alot of feeding. Do not underestimate how many of your fish it would eat with great pleasure..
Ben