I foster a lot of dogs from a shelter I volunteer for, plus I do a lot of dog sitting for friends and family. Sometimes, adults dog won't like each other and you have to keep that in mind. I have been lucky and only had a recent experience where a dog a was watching didn't get a long with my girl.
If you have a male, get a female, if you have female, get a male. It is best if everyone is nuetered. Identify the issues your current dogs have. My female is rawhide aggressive, doesn't like dogs in her face, won't tolerate any other dog going her cage. Rocco can go into Maia's cage, steal her rawhide, and get in her face but no other dogs can. My male does not read other dogs body language well and continues to play and he plays rough.
Introduce your dog(s) to the possible new one outside of your home, on neutral territory. Do the proper dog introduction things, loose leash, don't force it, etc. Let the dogs meet a couple times before bringing the new dog into your home. Make sure you establish each dogs own area and try to keep them out of the others. Don't leave the dogs alone in the house, yard, anywhere for a few days to weeks to make sure that their won't be any unforseen issues.
I think if you follow your instinct you will know if it will be a good match or not.