It is fine in my opinion OP.

If you spent 15 hours on the fight but took first timers every time someone left your party, you would never get much further than the last time. By taking only same experienced people you have much higher chance of actually continuing where you stopped. Even the best player in the world just cannot overcome true experience if he hasn't seen the fight yet.

You aren't obligated to sacrifice your own spare time for the benefit of others and you are entitled to state whatever requirements you want in the PF as long as they aren't against the law or ToS.
Sure, I believe you have some moral obligation to help your FC mates, friends or static members but definitely not strangers (my opinion). If you waited untill whole server experienced some fight, you would never clear it.

I feel somehow unbiased in this matter as I no longer use PF for such stuff (cba doing stuff like Thordan via PF when I have static full of nice people). So nope, I am no Party Finder elitist, if people want to accuse me of that.