Your puffers will most likely eat any snail and possibly any hermit crab you put in the tank. The idea of getting larger snails will help with the dogface not eating them, but with the valentini, it does not matter much what size you get. Your valentini has the mouth to just pick the snail or crab right out of its shell.
I have two puffers, a blue spot (same type as valentini) and a striped (same type as dogface) and they both eat the inverts I put in the tank. Now, I just put inverts into the tank to help them file their teeth because if you don't, their teeth may grow pretty large.
If your algae is that bad, stop using tap water (because this contains phosphates) and switch to RO water. Also, if you only have a dogface and valentini in the tank, slowly cut back on the amount of light you give the tank each day.