YOU are fun!

You and your other dogs! It sounds to me like she's got a pretty good social life just within the family

 Quote: Alyssa Myracle

I just don't understand the reasoning behind allowing dogs to play together.
It's not as though my dog's life is void of companionship or play- I provide that for her.
The only thing my dog can receive from other dogs, that I cannot provide, is a vicious attack, disease or bad habits. Any positive thing another dog can offer, I can easily provide myself.

I think it can be part of an exercise program. I can run my dog with the bike, and do some other structured exercises with her, but she's too big for wrestling, and I'm too slow for darting around the yard. These types of exercises help her coordination and reflexes, while most of the structured work with me foccues on just strength and areobics. However, I make this a very limited part of her exercise program, as I'd rather we spent more time together as a pack instead of involving outsiders, and these types of exercises are more prone to cause injury than the more structured work with me. It's not really natural for a pack to so easily accept outsiders. (I of course have plenty of human guests, Maggie is not expected to play with them though, they are for me to deal with, and for her to watch or ignore.)

 Quote: Ruth O'Leary

She hasn't met the other dogs before, I was just saying I'd make sure they were all relaxed before letting her offlead. I guess I just wanted the muzzle to make sure nothing bad could happen.

Um.... I'm sorry but... you really need to think this through better. You were planning to let her off lead? Knowing she has a history of fear aggression? Even with a muzzle on, what was your plan if the other dog frightened her and she bolted off? Even in a fenced area, a bolting dog is impossible to catch (trust me, I know). This would NOT be a good experience for her. I also think that a muzzle would only add to the strangeness and newness of the situation, and she would be much more concerned about the wierd thing on her face and the other dog looking strange with the muzzle on its face. Unless she is VERY used to being muzzled and seeing other dogs in muzzles and knows how to play with a muzzle on.

If you insist on making your fearful dog interact with strange dogs wearing strange facegear, at least have both dogs on a long line so that if anything goes south, or play gets too rough or uncontrolled, you can more easily grab them and get things back under control.

When I am introducing a new foster into the pack (which I do one at a time), both dogs are leashed and controlled, but I would not use muzzles. If your dog is aggressive enough to require a muzzle, perhaps it should not be interacting with other people's dogs (especially off leash!)

Your thinking here is backwards. Leash first, add muzzle if required. Not muzzle first, then leash if required.