I think you should be able to do the transfer all at once. I would premix the extra water, move the substrate, LR
(thats is where your beneficial bacteria are) water and fish. The only problem with this is stirring the substrate, it might cause some ammonia, but if you have pleny of extra premixed water for a PWC
, I think it can be done.
This might be about the only puffer that will feel comfortable in that size tank, Saddle Valentini Puffer and he will eat inverts and corals, as far as I know.