Well, I will definently not keep it in the 75 if it gets even to say 13". I've been asking people around and looking around for info on them and most places are saying they grow to about 6-8". If mine maxes out at 8" it should be fine, just what I think though. Right now I cannot afford a bigger ...

That's too many fish to start a tank and catfish catch ich somewhat easily.
Goldfish are coldwater fish while all the others are tropical. They should not be kept in the same tank, as they require different water temperatures.

Hm, but I've heard the average adult length is only about 8". 18" is in the wild and that's the MAX length. So say it grows to 8", what size tank would be 32 x 16 x 16?

Hm, I have one of them. Do you think a 75 gal is big enough for an adult?