Why not just wait until you can properly care for a goldfish before getting one. Which includes it having a companion of it's own type and not putting tropical fish in the tank. Two different types of fish IMHO. Keep your eye out for a cheaper second hand bigger tank and just get two.
I keep emerald cory cats with my 4 fancy goldfish, (long story). The only problem is making sure they get fed because my goldfish have learned to steal food from them, and goldfish will eat anything that will fit in their mouth. It's best though that you keep them with their own kind.