Turns out they can tolerate up to 1.0226 SG, hardy little guys! I decided to put in two MTS thats on the larger size, and they immediately burrowed and disappeared in the sand.
I did some research on that, and again, it's mixed experiences, some say puffers learn quickly how hard their shells, some say the snails never get caught since they bury in the sand and only come out at night for clean up, and another said their puff learned how to nip at their feet.
I figured I'd just remove them when the puff goes in, shouldn't be too hard since I can see where the sand has been dug (it's quite cute actually). Also because I read their tough shells could chip or break puffs' teeth and thats not a risk im willing to take, especially since my puff is still a juv. I might just move them to a new smaller tank and maybe keep them with some shrimp i want to breed, Neocardina can have sand as substrate too.
Water update after 75% WC
Amm - looks like 0.1
Nitrite - 2 (reduced from 8/9)
Didn't bother testing nitrates
After a couple hours, Nitrites went up to 3 - 4, Amm stayed the same
Will continue with WCs, thanks for the advice!