Yes, bigger tank is vital.
It doesn't have to say goldfish. Any of the NLS is fine. The Thera+A has enough garlic to kill some parasites so I recommend that one. Any of the 'regulars' are fine ('cichlid', 'discus', 'community', etc.).
No. No fish needs variety. Variety is a classic way to try to get proper nutrition that we have all been trained on. But the main goal is complete and balanced nutrition. NLS does this on its own and does it better on its own than with anything mixed in. I know this is the exact opposite of what most people think, but variety is not a nutrient and is not vital for nutrition. I have not found any other foods that I would trust to feed exclusively besides NLS.