If you get a 6' x 18" footprint, the tank will seem cramped when they grow up. The extra six inches of width makes a huge difference. I agree with those who said 180/220.
180/220 would be adequate IMO but keep in mind, the footprint size is more important than height, so the bigger the better. The extra 2' of a 240 would really help, but the cost of the tank will go up as well...
oscars arent really that mean the texas can get nasty if they are both males you might have to move them to 2 different tanks if they cant get along and i would say at least a 200 would work out better for them and give them the room so there might be less fighting between all the fish '