pufferers eat crabs... and some are piscivores... and if your talking about gsps or 8 s. they are tehcnically marine fish... and both outgrow a 10... so that is a verrry bad combo.
ok so how about the knight goby and a red claw crab. I have never delt with red or fidler crabs but its what my Wife and 7 year old think would be cool. I just think the crab sounds boring ass it gets and want something more in the tank.
There are dwarf puffers available that could survive a 10 gallon tank, but as MonsterMinus said, those puffer teeth are for cracking shells of crabs and mollusks, and taking bites out of other fish.
ok so puffer is out I get that. And the stupid crab need brackish water and its only a 10 gallon. Is there a good fish canidate I have not been able to find or is this a lost cause???
mudskippers come to mind.. but in that size tank i think the cran would eventually catch and eat it. and iir they need more then 10. imo your best off with a few crabs setup... and they do need land or they will find a way out, lol. nothing like chaseing a crab across the kitchen floor....