Breeds similar to ACDs.

Honestly, any herder is going to be prone to dog aggression, especially SSA - you're going to have to look into specific lines of the breed you want to find one less likely to produce DA dogs.

Very common in working line GSDs, common in BCs depending on lines, I know many aussies with aggression, too.

Add in the tension of a leash and if you don't have good obedience, there's a huge chance of the dog being a jerk.

It's a chance you take. :-/

From what I've heard, dog aggression isn't commonly found in koolies. That's the only herding breed I can think of :/

Circus (a Koolie) can be scrappy at times and I've had to separate fights between her and Pablo (my boyfriend's 2yo lab-mix). I say that he starts it, SO says that she starts it. Circus has NEVER had any aggression issues with Lugia and when it was just the two of them I left out chews, bones, toys all the time and they got along extremely well. I do notice Circus acting bitchy at Lugia from time to time and Lugia just walks away. But now that we're living with Pablo we have to put the bones and toys away unless they're being supervised. I honestly had no idea she was like this until I moved in with Pablo. She had never been in a dog fight before and now with Pablo they get into an altercation maybe once every 2-3 weeks.

Circus is definitely willing to fight a dog that has pissed her off. Fights have broken out between Circus and Pablo for the following reasons:
I'm petting both of them at the same time, Circus got too close to Pablo chewing a bone, I'm taking turns playing tug with the dogs, and if they bump each other it might start a brawl; Circus was stalking a squirrel and then Pablo shows up scaring it away and she might re-direct on him.

Another time an offleash mini poodle came charging at Circus barking, she pinned it to the ground snarling, but I separated them before that turned into a fight.

Other than that, Circus does fine with playgroups and stuff - I don't really consider her dog aggressive, but if she's around another dog that's kind of scrappy she'll feed off that energy and want to fight.

How serious re the fights? I have only had to break up two fights and that's because the other dog tried to escalate it and Josefina was intent on finishing it, there was blood in those two fights.

but I never break it up unless I absolutely have to, I believe they should work it out on their own if at all possible.

Aussies and mini aussies aren't prone to DA, IME at least. Also rough collies (lassie) don't usually have issues with DA.

I wouldn't even call ACDs really DA, dog selective, maybe but I think going as far as to say they are outright DA might be a little extreme.