So glad you came here to see which kind of pet might work well. Researching a new pet is a great step in happy pet keeping.
Two Goldfish need a 29 Long (minimum) or larger, a 40 gallon breeder tank would great. It is 36" wide, so is a 46G Bowfront. Both sizes would be great Goldfish homes. With a double stand you would have room for your other tanks too.
What kind are your goldfish? Some Goldfish get over 12 inches long and some can weigh a couple pounds! So the 6 gallon will not be a good home for them.
And to properly care for the goldfish a huge filter.
What size/type do you have now? Goldfish are huge ammonia producers. Because of this they are susceptible to Ammonia, and Nitrite poisoning.
Have you read about how the water needs BB (beneficial bacteria) to break down/process fish waste? Maintaining excellent water parameters for the fish are critical to good health.
Have you tested your water for Ammonia, Nitrites and Nitrates?. One of the symptoms are red/purple bruise looking spots, or red blotches and red streaks/veins, in the fins, this can seriously injure fish internally and sometimes, they survive but don't really recover due to physical damage internally.
How often do you do partial water changes?
Also your filter... I forget for sure but it should circulate the water 4-5x per hour, the gallon size of the tank for goldfish. Canister filters are ideal.
I know you asked about a new pet but it seems you care about your pets by asking questions, so I thought it to be wise to make sure you knew some basics on how to keep them well!
A nice idea for the 8 gallon might be Red Cherry Shrimp, and a nano fish called, Boraras Brigittae, "Chilis" It is a Micro Rasbora, so tiny it won't eat the baby shrimp! With plants and sand! Cherry shrimp come in different colors too, orange, yellow, blue, white and brown, though they usually cost more than the red ones!