Sorry about the typos in my last post. I am at work and rushed it off before a proper check :roll:
Goldfish need a lot of space so 3 would fit in a 3ft tank - minimum.
Sorry I know nothing about the cleaner you purchased so can't help there.
Whitespot remedies are easy to find at any LFS. Just follow the instructions on the bottle.
Sexing goldfish isn't always easy. Look down on them from above. The males are more streamlined. Females tend to bulge slightly to one side making their tails look slightly out of line. Also, when about to spawn, males develop white spots around the gill plates.
When I mentioned the fancier types of goldfish, I meant veiltails or fantails.
For other types of fish you would have to go tropical. You can get gold gouramis or black widow tetras as an example. Google these and see what you think. You would have to buy a heaterstat if you decided to do this.