Live rock is great stuff and gsps love to scan it for little critters but you do need full saltwater for this. I wouldn't recommending adding any until 1.020 or higher salinity. Full saltwater may be a little daunting at first but is easier maintenance (less water changes due to protein skimming means less money on salt) and your puff will thank you for it.
Forget other advice about puffers you don't hear here - Pufferpunk
I did find a way to help my one puffer from constantly swimming up and down, I found another puffer same size and about same age. He is just the happiest now, they both swim together most of the time.
I did put some gold mollies in there as well.