I would not go to 4 GF till you have the bigger tank.
The bridge idea sounds interesting. I get that the total gallons is larger than the main tank.
I just don't know if putting fish in a 29g would stunt their growth.
I normally recommend 29g for one and 40-55g for 2.
They do a majority of their growing in the first year, once they are stunted, they may never live to their full potential.
I think 2 will be ok for maybe 3 months ?
Why fancy goldfish require big tanks- visual perspective
YouTube user Solid Gold has some great vids.
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