this is how you would feed mussels and clams for your little guys. You open and then they scrap their teeth as they jerk out the meat.
http://www.youtube.com/watch?v=XzONcFdiGhI
if you had a bigger puffer they could just crush the shells themselves.
I would stay away from the pellets, just because real food is so much more natural, cheaper and better..and fun to feed

For things like blue snow crab legs...you use a hammer and crush them a bit and they will chew on the cracked shell to get to the meat.
just to get you started Walmart sells a bag of mussels for about $3.50.
Make sure to quarantine snails when you get them from the store to make sure they are healthy.