Whether or not this is PTSD is for a psychologist or psychiatrist to decide. However, yes, it does sound like certain behaviors trigger you, and, YES, bullying can be and is traumatizing. It also sounds like the bullying at work triggered stuff from your past and so it's sort of a compounding of trauma from your stepmother. I had almost the same sort of compounding with a female bully at an employer who reminded me of my mother.

I would encourage you to allow your new coworkers to show you who they are and take them on your face value. It does sound like you experience some symptoms of PTSD, like correlating them (who you don't know yet) with people who treated you poorly. That's a little bit of flashback behavior - operating in an old reality when you're in a new reality.

I think you should see a therapist and get some help with distinguishing these new coworkers from your former coworkers. Always check back in with the evidence. Don't assume any behavior is related to anything else.

You could also take on the mantra of being an example for them - since the cliques and gossiping were upsetting - that you want to model kindness and uplifting each other as a team. Sometimes teams devolve into that high school behavior because no one really shows a model of behaving better.

I don't think you're prejudiced. I think there is some trauma that's clouding judgement, and I do think a therapist could help you work through it and thrive.