not too sure if you understood what i was asking, since you replied with "until the bacteria can finally get used to him". i do hope you are prepared to do the necessary water changes to avoid a mini cycle.
what i was asking was if the current stock in the tank (gambusias, goby, and shrimps) create a waste amount equal to the waste amount of one puffer. if you were to remove all the current tank inhabitants when you get the puffer, would the waste produced by the puffer be equal or near equal? (so as to avoid ammonia spikes/mini cycle).
also, even though members on the other forum you've posted on suggest a varied diet, you've mentioned you will be feeding brine shrimp. this is not a nutritious enough diet for any fish. please reconsider this.
edit - sorry if this sounded a lil nasty or cold hearted, but i wouldn't want to see your future puffer suffer at all.