Cricket is a subsidiary of AT&T and uses AT&T towers and frequencies. You should be perfectly fine buying an AT&T model of S7 Edge and using it on Cricket. There should be no need to SIM-unlock it from my understanding because it is using AT&T. However, it wouldn't hurt if you are buying used to make sure the seller has SIM-unlocked it just in case.
My understanding is that the non-carrier model of the S7 Edge has all the same frequencies (this is not always the case). So if you find a non-carrier model that should work fine, too. That would have the added advantage of not having all the bloatware, but usually non-carrier models are a little more expensive in the used market.