Hi, welcome. I own a CO, tho I've owned up to five adults in the past and have raised 2 litters. It is pretty apparent at as young as 4 weeks of age which puppy will be the more aggressive guardian, so an experienced breeder should be able to pick the right pup for you. However, I would only buy a CO from very stable parents - and I wouldn't take a breeder's word for that - I'd get out and see them myself. If that's not possible, then consider passing on the breed. Their is a LOT of shit-nerved CO out there, and they represent a HUGE liability.
As for keeping them in suburbia, I think it would be tough, but it can be done. Female CO bark A LOT, so keep that in mind... However, male COs can be a lot harder to handle than females.
Generally speaking, I have found CO to be pretty easy dogs. They listen to their owners better than most LGD and they are clean and typically chill in the house. But, as others have said, it's not the 90% of the time you have to worry about them, it's the 10% of the time when they are guarding that you have to worry. If your fence cannot contain your CO, no amount of exercise, shocking, or training will keep them from getting over the fence to deal witha potential threat - and remember what you see as a threat and what they see as a threat can be very different.
I have owned CO who didn't challenge fencing, and that were friendly to guests on our property, and then I have owned CO who would jump a 7' fence and were totally intolerant of any stranger on our property. This is why you need to know the parents of the pup you get and buy from an experienced breeder.
CO can be both the most scary and unpleasant dog you will ever encounter and the most loving and gentle dog - it just depends on what side of the spectrum you fall: enemy or friend.
You own a GSD, you should know, CO are nothing like GSD. CO are not biddable the way a GSD is, they also are not "handler hard" the way a GSD is - you can't beat your CO and have him still like you - they are soft with their owners, you need to be soft but firm with them. Most GSDs will take a slap from their owner and ask for another - that is not how CO are.
Now, having written all that, keep in mind, this breed is full of hype. You gotta cut through the hype - they are dogs, not bear. They learn like any other dogs breed - they love treats and cookies, and being pet by their owner - like any other dog... They are just a LOT of dog with a natural guardian instinct that lies very close to the surface.
When picking a breeder - I would be wary of any breeder of any breed that spends a lot of time talking negatively about other breeders. You gotta ask: why spend so much time deflecting? Also, I would be very suspicious of gimmicks *cough* ADR * cough* ... Dogs are complex beings, with complex behavioral confirmation, and personalities... These things cannot be quantified in a rating system - no more than they could be in human beings.
Good luck.
