After reading your thread I was horrified at the amount of fish you said you had in a 70l tank when you were saying it looked bare, I had visions of the fish being squashed cheek to cheek.
Admittedly 70g is a lot better! lol, you still do have some fish that will require rehoming as they will outgrow the tank. I think the general rule is that the tank should be 4x longer than than the full length of an adult fish you wish to keep in it, this is just a general rule, so if yours is 4ft, then you cant keep fish in it that will exceed 1ft (12") in length, it just isnt fair having them cramped in that small space 24-7.
I'd seriously do some research regarding all the fish you currently have before even contemplating adding any more. Also I'm sure oscars need more than 70g, someone correct me if I'm wrong but i thought they did.
Andy