I think Assassin snails burrow just like the MTS.
I am not sure that a puffer will eat MTS, they may be too hard.
I have a pair of Kribensis in my 55. My female is only a little more than 2", so she might end up as lunch, but my male is 4", so maybe you could get a male or two? It's a shame, because my female is way more colorful and outgoing than my male, who is skittish and picks on my angels. She is my favorite fish. Before they went in, the tank was covered in MTS. A week later they were almost all gone. There a few in there now, a couple of months later, but only a handful, and never out in the open.