I agree with gardenandcats - pick a high prize item to tempt him. I would also put new kitty in quarantine for a few weeks. Do you have a spare bedroom that she could stay in? Also, your shelter kitty may have (or may develop) a kitty cold from staying in the shelter. It's a common occurance in shelter cats/kitties. Your resident guy needs to know he is still number one. He should get full run of the house. They can used to each other's smells from under the door. Then put some stinky tuna (or what he likes) in a bowl by the door where the kitty is. She can have a treat on her side too. That way your guy is thinking "yum tuna, and hey, what's this other cat smell,...well who cares...yum tuna". The idea is for him to associate good things with the new member. It doesn't have to be food - it can be a special toy he likes - mine are addicted to feather toys & the laser light. Anything you can do so that he associates good thing with the new kitty. Also lavish him a lot with praise. New kitty should play second fiddle for a while, you're guy has to know he is still top cat.