I understand about being overly cautious with the cat/grey introduction, I was paranoid with my first grey I just brought home a new dog over the weekend, so this is fresh for me. I would muzzle your grey, put him on leash, and let your cat walk around in the room. If your grey shows little or no interest in the cat (some curiosity is normal), the next "trial" you can drop the leash (but leave it attached) and see how they interact. Having a confident cat is a huge plus, and I assume you got a dog that was tested as cat "safe". It is always good to be cautious though, each cat and each new situation is different.
As for the growling, I agree that he was probably uncertain or frightened. A growl is a warning, and I do not correct for that. Also, while the man may have been perfectly harmless, I always trust my dog's instincts. Greyhounds as a breed are not known to be protective, but some individuals are, my Fletcher was very protective of me and his girls.
Sounds like things are going well! Congratulations!