I'd recommend just keeping one I've had goldfish for over 20years. And I would not do more than one in a 20gal. With bigger tanks you can get away with I higher stocking level but in a tank less than 40 gal
I like to keep it a minimum of 15gal/fish and no smaller than 20gal.
All that said you do have some openings. You can get a small bread of the cost of such as bristlenose plecostomus or rubber lip plecostomus.. You could easily do one of those varieties in the tank with the goldfish.
I'd recommend adding live plants if possible.
Also you need a good filter with goldfish for a 20gal tank I'd get a HOB
rated for 2x the volume. A small canister would work also.
I have also saw goldfish do well with platys before but do know the goldfish will eat the babies. There are a few other options but as you get more complicated you have to be more careful and watch closer so you can catch any problems.