I wouldn't put any more oscars in your 160 gallon tank. regarding the 55 gallon tank, yes, as long as you weren't planning on keeping the Oscars in the smaller tank permanently, you could easily grow them to seven or 8 inches ready for a bigger tank.
If you are planning on getting six Oscars in the hope of obtaining a pair, remember that you'll have to wait untill they reach maturity, which will probably be more than a year. By that time, they could be 10 inches, even more. You're going to have a real hard time maintaining water quality in the 160 gallon tank. I did it here, but I had the 300 gallon tank that I could transfer some of them into. Personally, I would do it with three Oscars if you haven't got a really big tank. Even with three Oscars, you should have at least one male, or female