I think (although not positive) that FIPS has to do with how the radio handles the encryption. How the keys are stored, how vulnerable they are, do the keys zeroize if the radio is tampered with, etc.
I believe a FIPS and non-FIPS radio will talk to each other if they have the same key. Although if your agency required FIPS compliance, you wouldn't be sharing your key into a non-FIPS radio.