I agree that saying "she just wasn't a good fit" would be sufficient if you don't want to go into detail.

However, if it were me, I would want to go into detail -- probably not in the first few sessions (because I'd be absurdly worried about making the new T think that I am high-maintenance... even though I definitely am, lol), but eventually... because it's *your* therapy, and if the old T acted unprofessionally (which, based on your previous posts, she definitely did), you have every right to process the effect that her lack of professionalism had on you.

I dunno about therapists, but as a doctor, I'm used to hearing patients' complaints about my colleagues; I've gotten the hang of listening and validating without saying something unprofessional myself (even if I personally know that that particular colleague is an utter fool). Heck, the new T may very well know that old-T is a basket case on wheels. And it'd be a huuuuge HIPAA violation for the new T to tell the old T what you said, and I can't imagine the new T would file any kind of report on the old T unless the old one did something completely egregious and illegal.

Do whatever you're comfortable with, of course, but if I were in your situation I probably wouldn't be able to keep it at "she wasn't a good fit."