Aww poor Sissy. I wonder what gets her dander up? Did this just start after amputation?  If so, this is just a guess, but she could be in come kind of pain and feels vulnerable around other animals? Has she been assessed by a rehab therapist? If not, I would make that my next move.

As for the harness, well as a front leg amputee, Sissy is just going to walk faster than ever, there's no real "heeling" with a front-leg Tripawd. They need the speed for balance and momentum and any of the no-pull harnesses on the market could do more harm than good to a dog with a compromised gait like hers. 

However all is not lost. There's no reason why you couldn't find a happy compromise. You walk a little faster while she tries her best to just take it a notch slower on walks. Have you tried repeating any leash training you did as a puppy? You may want to revisit leash training tactics but allow some leeeway for her need for speed.