As for the poop schedule being messed up...don't worry. Remember also, that he might get soft stool or a little diarrhea from food changes.
Also - I am a big fan of harnesses in addition to collars. New greyhounds can get scared on a leash and can slip collars easily - as I learned once. The harness is a great backup. When I walk mine, it's with two leashes each...one to a collar and one to a harness. If I had to use only one, I would suggest the harness as long as he isn't a dog who pulls a lot.
Oh...and mine love to pick their own toys at the store!