I had a couple small guppies in with my 5 DP's and after a week or so I never saw those guppies again. I find those pygmy cories to be too hard to keep alive that I wouldnt want to add another possible stresser.
all puffers I've seen are very avid fin nippers, to the point where the victim usually doesn't have fins. Not to say that they all are, just the ones i've experienced. Being that corys are for the most part timid and non-agressive, I personally wouldn't try it. But if you are set on it, you could throw it in and take it out at any sign of abuse. Another route you could go would be loaches. I'm not sure which ones would work for you specifically, but i'm sure someone could give you more info. Tank dimensions and tankmates could help us help you find an alternative if you are looking for one.