As far as the ACD temperament, unfortunately around here there have been some really nasty ones. At the vet clinic we had a few clients with ACD's and some of them had to be muzzled for everything. And I had one particularly memorable dog come through my class who had no socialization as a young dog, had been allowed to get away with everything, and then when it started being dominant aggressive as an adult they decided to come to class. The dog bit me several times.
They may not be aggrissive, they can just be stressed. Vet clinics are very stressing for a lot of dog's, they muzzled Charlie just in case because he kept trying to run from them, but if he is at a park he is great with people. Even Patch (The nicest dog we've ever owned! She was a mutt) was very nervous at the vet even though, seriously, she was the friendliest dog i've ever seen.