welcome on board!
first, having more that two oscar in 75 gal tank
is overstocking already. you are prone to a lot of
problem later on because oscar is a fast growing huge
fish.
difference in sizes is not much of an issue. juvenile
size eat as much as they can every day. grown up, once
full do not take much foods in a day or two. time will
come that the small one will catch up with the big one.
i was told lately by a breeder in my place that female
usually out grown the male and two or more female with one
male is not an issue. however, if you happen to have two or
more male, you will be in trouble. they will fight for supremacy.
i have not experience something like that because i always buy
in tandem. lucky enough, i always end up with pair.