The shelter I adopted Duncan from told me that he wasnt' good with other dogs either and that I couldn't have him because my roommate had a dog. I kept pushing the issue with them however and even brought Jack in to the shelter to meet Duncan. They were fine together! Turns out they were basing their "opinion" that he wasn't good with dogs on the fact that every time they took him out of his cage, which was the last one in a long hallway with about 20 cages of large dogs on both sides, he would lunge and bark at the dogs in the cages! Well, they were all lunging and barking in the cages at him in the hallway! And he had been there a month in that back cage and was probably scared, excited, nervous, all sorts of emotions...
So check it our for yourself. Don't just take their word for it. And hey, all dogs can change with the right home and TLC right!