I would say no only because the Oscar will grow way faster then the others and once they get to the point that they can eat the others they most likely will. I'd say grow the others out a bit and then add the oscar later.........
I agree with the other responses, I'm also worried that the oscar will get picked on as they are less aggressive than JDs and Cons, some African Cichlids are downright mean too.
Perfect size for having an Oscar of that age, for a year maybe 2... As for selection, Good starter.. everyone Has their own opinion and just look for what you want.. It's your tank your fish, And everyones tank is too small if you look wehre the fish are taken from LOL...
I raised My O with other SA's and Ca's and Africans as well.. I raised him to 9" in a 40 gallon tank.. I had 2 to three cons as well as a Mono,a couple africans, A snowflake eel, Pleco,And guess what after 4 and a half years Is now in a 150 of course the past 6 months it was a 55.. But hey as your fish grow you'll learn and then you'll go bigger..
I believe you wont have too many probs with agression as long as they are pretty much all juvies.
I'm now looking for an even bigger tank as my personal prefrence has grown as well as my Fish But do with what you have.