Yes I'd also say have a safeword if you're worried about being triggered. When you're triggered how bad does it get? Does he know what to do if you're triggered? Is there a plan in place for this? He should know all this in case something happens.
When I get triggered everything stops immediately. We have a code so he knows where my head & body is & what to do if it's really bad. Dealing with an old memory or flashback can be very difficult & needs to be addressed asap before it ruins the relationship.

Hair pulling is on a sliding scale that can be gentle all the way up to rough too so u need to know your threshold. I can't stand to have my hair pulled from the top of my scalp so it's off limits. Anywhere else, he knows is ok. So experiment.

Deep throat can take a lot of practice for both people if it doesn't come naturally for a woman...or a man. It's all about positions & angles to really get it where it's supposed to be. If you have a natural gag reflex you'll have to work on just getting over that before you go any further & that's just practice on your own. There are ways to make it work. Educate yourself on all this. Makes it easier to understand.

Slapping can very very sensual or totally freak you out. That you will have to explore where your threshold is. And I think flinching is a natural reflex especially when you don't know when it's coming. Does it have to be on your face? And what purpose will it have? What outcome is expected? This can make a big difference? I do not expect a slap on the face for punishment, but to get my attention regarding my posture or my mouthiness. Why does he wanto slap you? How will you feel?

But I think you've talked about bdsm before if I'm correct & these r all things that should be gone over before ever entering the bedroom bec so much can go wrong very quickly & then people get hurt, feelings & emotions. Have these conversations in the light of day with a clear head so you & he know your soft limits & definitely the hard limits.
Many use 3 safe words. Green, yellow, red so the other person knows where your head is & where limits are. Don't be afraid to use them.
I know some boast "I've never safeworded!" Yeah great, but I safeworded when my fingers were being crushed between a couch & a wall! It freakin hurt...& I like pain, but I didn't like this...& then we laughed about it!

Plus all these things take time to get used to. It's not like porn where everything is fantastic the first time. Talk & talk some more. Even if it's embarrassing, but you need to feel comfortable & also confident. Do you trust this person.
FWIW do you mean doggy style or backdoor.
Anyway...have fun! Protected fun lol!