> she is a real scaredy cat otherwise
This here is the crucial bit of information! This is bullying type behaviour, not bossiness. It can become very intense in collies. And bullying, just as in people, is based in insecurity and fear: trying to hide these things and trying to control others. As a need to control a situation can also be a big collie thing that can make it worse. I'm working through similar behaviour with Willow, who will badger the faces of some of my dogs on a walk to the nth degree and it drives us all insane. It looks like she's just playing if there's no-one else around, but bring in a dog from outside the pack, which stresses her, and the behaviour intensifies instantly and goes far beyond anything I can do to stop it at present (she'd had bad experience with dogs, on top of not being socialised properly before I got her). Phoebe too can be a bully indoors, but hers comes out more in guarding, especially when toys are involved. If the others are playing though she will sometimes go and try to stop them by barking or standing over them growling. All of which I've also worked through with my collie cross!
Your girl does sound a fair bit worse though. And once these guys are in a habit, it can be serious work to stop it. You'll need to start piling on the rewards - anything and everything, no matter how small, that
isn't bullying in some way, reward it. And I mean everything - and nothing counts! By that I mean if one walks past her at home and she doesn't do anything, reward it. Nothing is a very rewardable behaviour for these dogs! That's what I did with Phoebe to start with - she'd gotten to a point that any time Linc came upstairs, she would fly at him and chase him back down (he's not one for walking away if a dog is guarding a chew, won't take it, but won't bog off and she has never forgotten it). So I started rewarding her any time he was near and she did nothing. Now she'll ignore him if he comes upstairs, and every now and then, I catch her sniffing a bit of him with her little fluffy-happy-ears on and her tail swishing gently. Only when he's not looking though :-P
Out and about is much harder; but I would start on lead. Your aim needs to be to not give her a chance to do the bullying again, which is going to be the hardest bit because of habit and speed! So on-lead will give you a good chance. If need be, alternate having her on and the others on so she can still get a run. Then the same rules apply: anything that isn't bullying gets rewarded, and heavily. Work on keeping her attention and reward her for giving it; anytime she looks away from the others is an excellent choice as she'll start to choose to look away rather than focus on them.
One thing I ended up doing with Willow was to use the others to train her; not sure how well it would work for you but worth mentioning. She will lay down up ahead, waiting for one of them to reach her so she can pounce and start badgering the hell out of them. They got wise to this pretty quickly but as she'd chase if they ran, started hesitating, not knowing what to do. So I took advantage of it. As soon as I saw them slow down, I called them back, and threw a couple of treats on the ground beside me for them. That gave me a chance to move ahead of them and move her on with hand signals (hands spread apart as a sort of block to begin with, which has now become one open palm which means 'turn around and go'). I rewarded her for doing it. They quickly learned to turn round and come to me when they spotted her, and she quickly learned that she wasn't going to get to badger. Now, they do it all themselves: they stop, she moves on, they continue and it's all peaceful.
Badgering while running around is still an issue so she still goes on lead a lot; while she's on I am still mostly rewarding her for looking away from them and at me. If I keep her on at the start of the walk for 5 minutes and do that, I can then let her off and control her. It used to be 10-15 minutes, so she is improving. Other dogs are still a trigger but we don't see many so that's taking longer to sort.
A good 'leave' can help too, I had to teach her that one specifically for Paige when she was put on lead-walks only because I didn't have any room for maneuvre for the sending on thing - just remember to keep it light and happy as any stress added will intensify the bullying. This is primarily what I did with the cross, River - she was raised in her first year to be a bully by her first owners who found it funny, so I've had my work cut out with her. What I focused on with her was simply calling her away when I could see she was about to start (hers has always surfaced when other dogs are playing), with real gusto, really squeaky/happy voice and making myself exciting then really piling on the rewards when she did come away. It took a little while but it's worked - she hasn't tried in a long, long time and the worst she'll do now is just sit and bark at them, and I can call her away from doing that no problem.
Just one other note: medical problems may be a factor. I didn't know until last year but River has a chronic prolapsed disc, which is pressing on nerve roots causing sciatica-type pain as well as achiness; but unless you know what you're looking for in her gait, you'd never know it was a problem. It wasn't triggered by an accident, it's just one of those things. The first symptom I saw was her refusing to do contact obstacles at agility once or twice - from there we found HD, then when that didn't improve, an MRI found the disc. She also has congenital narrowing of the canals through which those nerve roots run so they're under pressure from that too. All of which was having a huge impact on her tolerance levels - with acupuncture, controlled exercise and pain relief, her bullying improved immensely. Phoebe has a longstanding shoulder injury (ran into an iron bench 2 years ago) which I suspect may be exacerbating things, again you wouldn't know it was there unless you knew it was there if that makes sense! I only realised it was still an issue last time I clipped her off, as the vibration on that side was making her flinch and pull away. I'm trying different natural painkillers for now before we brave the vets (Phoebe has many phobias not the least of which is the car, so the vet is only done when absolutely unavoidable).