So I kind of think those are two different problems, but they're solved mostly the same way. The thing with her bolting after deer and squirrel is a prey drive thing, clearly--chasing is soooo much fun that even if she hears you, you can't possibly compete with the fun of chasing a critter! The dog thing could be partially prey drive, but I also think it's likely that she just has a bit of a hole in her socialization. If she's only reacting after they growl, that makes me think that she's running up and being rude, then when they give her a "back off" cue she interprets that as a threat or a challenge and reacts aggressively. It could be just manifesting with small dogs for a couple of reasons; most likely in my mind is that she feels safe challenging a small dog but not a larger one.
Agree totally with Mary Beth. No off-leash work for the time being, and if she does see something she wants to chase, do whatever it takes to get her attention back on you. I've even taken off in the opposite direction for a few steps when nothing else worked. Be really focused on her though and try to get to her as soon as she notices it, not once she's already in chase mode. Never let her indulge in chasing small animals, because it is very rewarding and it's going to be hard for you to compete with that! However, be sure you do give her an outlet for her prey drive by playing tug and fetch (the latter in a safe enclosed area, or on a 30' lead). Saying "don't chase anything!" doesn't really work for a dog with a high prey drive, but setting firm rules about what she can chase (toys, for example) will work if you play with her often enough.
Also, for teaching focus on the handler, I really like clicker training (well, I like it anyway, but you know). Hector used to be terrible about paying attention to everything but me, so what I do is carry a treat in my hand as we walked and whenever he glanced at me, I'd click and treat. This is in addition to the frequent direction changes, etc. that Mary Beth outlined above. He learned that paying attention to me and watching me is A Good Thing and learned to focus on me even in distracting situations very quickly. Now he'll usually look to me first when he sees something he wants to investigate. Once we established that looking at me would be rewarded, I started adding in saying his name quietly (not like a whisper, just a normal tone) and rewarding for that. The benefit of that is on the occasions he sees something that is just too too exciting, I just say his name and he looks at me and sits. Easy peasy.
Regarding the small dogs, what situations are you encountering them in? On-leash or off? If they're growling to get her to back off, that sounds to me like she might be being rude. A lot of dogs don't have great manners when meeting new dogs and run right up into their faces...this is kind of like a total stranger racing up to you and giving you a bear hug. Some people might not mind, but a lot will be scared or annoyed.
For this, I would definitely recommend working with a trainer. However, an easy thing you can start with is teaching her to ignore other dogs as you pass them on your walks. It's easiest if you can get a friend to help so you can eventually get really close, but even just rewarding her from ignoring small dogs at a distance will get you started.
Basically, the fix is the same as for the squirrels and deer--make her focus on you, and reward her for doing so. Start at a comfortable distance where you can keep her attention on you fairly easily (close enough that she notices the other dog, far enough that it's not that exciting yet--it might take some experimentation to figure this out). As she learns to ignore the dog, gradually move closer.
If you have a friend with a friendly small dog who will help, you can even eventually let her go up and sniff the dog. Remove her if she gets too in its face or aggressive, and back up. Reward her if she gives a basic sniff and then looks at you. Don't expect a long interaction, and try to keep things positive. The ideal is for the little dog to not feel threatened enough to growl, and for your girl to learn that approaching new dogs calmly and sniffing politely is the best response.
This does actually work, btw, as long as it is just a socialization problem. Hector had problems that sound almost identical to your dog's issues with small dogs when I first got him. After working with him for a few months, he learned to be very polite with other dogs, to the point that experienced dog people have often commented on it (particularly since if he does meet a rude one that likes to play, he's all over it LOL).
However, it's best to work with a trainer because this can be a long process, and because if your dog just naturally has a dog aggressive personality, she may not ever be trustworthy with small dogs (and you may see it shift to include larger dogs as she matures). A trainer can help you determine if it's something that can be completely fixed or if you need to do a mixture of training and management to keep her away from other dogs.
The rowdy dogs:
Hector-2 y/o GSD (mix?) rescue
Scooter-12 y/o ACD/Border Collie mix
Bandit-8 y/o ACD
Wooby-14 y/o ACD
Abutiu "Abi"-ACD puppy and hopeful future SAR dog!