Deciding on another rescue dog

Question
A month ago I got a 3 - 4 month old male labrador from a rescue centre. I have now seen a female neopolitan mastiff possibly around 2 years old, she is also from a rescue centre. I've been told that they won't let her go with other dogs as she is a dominant dog. Do you think the 2 dogs together would be a serious problem or are the rescue centre just being over precautious?

Answer
Well it depends on how dominant your dog is,i too have a dominant dog,but was able to find a very submissive dog for her-so if your dog is submissive it might work,but two dominant dogs is asking for trouble,so that's why they may have said that,they don't want dog fights