I think some things are slightly flexible. Like if I get anyone interested in bondage, I tell them I won't let them physically restrain my limbs (so I can't escape, basically) until we've played together for a little while, and then as they become more regular and I'm more relaxed around them I'll happily negotiate this sort of play (though in my private kink life I did pick up the nickname 'Houdini', so even if I did find myself in a sticky situation, I could probably escape pretty easily). It's just for my own peace of mind.
As for other stuff, if someone I've previously seen asks for a service I don't offer, I quote them a higher price. Anyone else I say I won't do that, but if someone I've seen says "would you mind doing watersports?" I'd say "you know I don't offer that, so it would have to be significantly more than my usual rates for me to consider that" if they still press, then I quote them I high price. I've yet to have someone then take me up on that higher rate, though.
That being said, there are some things that money can't buy from me. I won't ever bareback. I won't do hardsports. I won't do anything that would tear the flesh. I won't do anything obviously illegal (animals, corpses, children etc etc etc). However, anything that doesn't make me instantly want to puke at, I'll consider at my own discretion.