haha, i have a crayfish in my other tank. he's way too big to even think about putting in a 3gal even if he would be okay to live with my puffer.
i've never seen any of those shrimp you mentioned czcz, and i've been looking in several places. the only shrimp i've seen around me are ghost shrimp and bamboo shrimp, which are pretty big. the only place i haven't checked is petco... do you think they would have them? if not, i'll talk to the owner of my favourite LFS
to see if she could order some for me. (how much do they cost roughly? petsmart sells bamboo shrimp for $12, which i think is absurd and wouldn't want to pay that much for an even smaller shrimp)
i was thinking about getting one ghost shrimp just to see if my puffer would tolerate it or not. he seems pretty docile, it takes him a couple days to even go after snails when i put them in there, but you never know.