I think you are headed in the right direction with what you are doing in requesting a more submissive dog. I have a male and female and they have always gotten along wonderfully. I've had two males before and they got along well although had a few minor tiffs. My male is highly submissive and my female a little more dominant. I think it works best to match them up that way if possible. If you know your boy is a little more the dominant type, going with a more submissive dog is your best chance. An older dog may also be slower to accept a new member to the "pack" but if you get a dog that isn't too pushy and backs down easily I would think he could adjust.

Taking him to the shelter to meet isn't a bad idea although I think it's always important to keep in mind that two dogs meeting on neutral ground may react differently than when they go into one dog's home and territory. I think if you watch closely and take the precautions you are taking, more than likely things will go pretty smoothly. Best wishes!