A lot of people think rescue dogs are problem dogs but it's simply not always the case. The majority of dogs in rescue have no problems whatsoever. I think saying dogs from a breeder are always perfect is a little misleading also - as the majority of breeders that advertise are backyard breeders who wouldn't know good temperament and health if it bit them on the bum. A dog from a
reputable and responsible breeder will have most chance of being of sound temperament - I totally agree with that.
But what about pups in rescue? They're just as much a blank canvas as non-rescue dogs in most cases. And then there's hand ins - dogs whose owners have had to sadly give them up due to personal problems, where the dog is well trained and a wonderful companion who's been very much loved and well cared for.
So saying rescue dogs are all unknowns and not worth risking with kids and other dogs is terribly OTT and a complete myth. Yes - many rescue dogs will have problems, some small (housetraining perhaps), some bigger - but there are ways of finding your perfect companion through rescue. Always go to a rescue who match a dog to your needs, instead of allowing you to pick whatever you want. Preferably one where they use foster homes so know how the dogs behave with kids/cats/other dogs. And only a rescue who will give you back up should there be any problems - whether that's with advice or help training, or by taking the dog back should any bigger issues come about.
At the end of the day no dog is going to be 100% perfect. Even the healthiest well bred dogs from good breeders have been known to have behavioural/health issues - and 90% of a dog's make up is how they are raised from there on. If you have the time and the space, and aren't just expecting a dog to come to you ready made and perfect - then why not a rescue? If there's no real reason why not to - why rule it out?
I would however be worried about homing a small breed with a GSD that's been known to fight with other dogs in the past. The boston terrier might not be as lucky as the Great Dane was, even if the GSD didn't instigate it - it could do serious damage to a small breed if it took a disliking to it (you've said yourself he'll only accept small dogs - which to me leads me to think it was him starting the problems with the Dane?).
At the end of the day - if your GSD doesn't like other dogs, what's to say he'll know the difference between a rescue and a dog from a breeder? I think you probably need to think a bit more about the possible consequences. We've been where you are - we tried a couple of dogs with Sky in the past - she seemed to bond very well with Badger, and lived happily with him for 2 years, then one day couldn't cope with his bounciness any more - and snapped out at him. We had to rehome him, as you did your Great dane. Do you really want that to happen again?