Sorry it took me a while to get back to you....internet was down at my apartment.
With Maggie and fosters I've found slow intros, clear rules, and lots of training and exercise time results in the happiest dogs. Slow intros actually are quite fast now, but my first few fosters took about 10 days to integrate into the household; my most recent foster took all of an hour.
Maggie is anxious around new dogs which is what triggers aggression - I use an herbal supplement when we go to big dog events or dog parks and she is much less reactive. Given this anxiety, dogs meet Maggie at our home rather than neutral turf, like most dogs should do better with. When I lived in ahouse with a yard, fosters were allowed to live parallel lives for about 2 days before Maggie saw them - they were in the yard when she was inside and vice versa. Eventually, Maggie would be allowed to see the foster thru the baby gate, then while crated next to the foster and eating dinner, then on a parallel walk, and eventually loose in the yard and house. Maggie has never had a major problem with a foster using this method - she woudl correct them as needed, but was never bullying or aggressive with them; new dogs met off the property w/o parallel walking generally see lots of Maggie's teeth, so the difference is stark.
Maggie is full of contradictions overall - she has the ability to be a fair leader, happy playmate, and good role model for other dogs, but she can also be a very insecure dog in interactions with fellow canines and show a lot of the "beta bully" behavior described in another post.
One thing I do when looking at fosters is select the ones I gravitate toward - oddly enough my choices often mirror Maggie's. Herding breeds are my favorites and they play/communicate like Maggie so they fit in well. The one non-herding dog I fostered failed horribly: stress diarrhea was one result and Maggie was much more of a bully with him; he also didn't heed any of her subtle (and even the not so subtle) cues to back off and he was just way more over the top about interactions than Maggie prefers.
Sasha, the current foster who broke the record for good intros and may be staying, is a herding mix, 6mo, female, intact (for now), and good at flipping between "dominant" and "submisive/appeasing/shy" with other dogs. She and Maggie play very roughly and share things equally - sometimes Maggie steals from Sasha and vice versa. Maggie does guard high value toys, but not me or water like she has done with dogs in the past.
I would highly recommend fostering first so you can "try out" different types and personalities of dogs as there is no cut and dried way of picking a housemate for a guardy dog. 
Maggie has not read the manual and I doubt she's the only one. 
I have noticed that Maggie has a pattern in behavior with fosters, length of each stage depends on the individual dog. It is as follows:
1: Initial wariness, hypervigilance - this is the stage I try to smooth out with the slow intros as if I push too fast here, she's more likely to "correct" a dog more harshly for minor issues.
2: Cautious optimism - this occurs once the dogs are going on parallel walks or first in the yard. generally marked by hesitant play behavior, but guarding is still quite possible
3: Playful, but still tense - marked by lots of over the top play, rough play that has an edge of sorts, less guarding possibilities
4: Order sorting - generally seen after a few weeks, Maggie often seems a bit standoffish and depressed
5: Resolution - after at least a month of foster dog's presence, Maggie is suddenly back to her normal self and all is well.
Erin, the Amazing Maggie Mae, and Super Puppy Ziva
