Rescue dog - well done!
Looks like you will get lots of advice on here. Main thing is to be firm, assertive and calm at all times. You are the pack leader so you have responsibility and your dog can relax and enjoy life.
Not sure what the harness is all about - it's not a good idea to let your dog pull you along - you should always be slightly in front of him. On the other hand, if the harness is being used to get him to pull something else along (ie he is working), that's a great idea and the dog will love it.
He looks like a great dog - enjoy.