how hard is too hard when introducing?

You two need to talk about the Boundaries before you start anything Rough. Really talk, about your expectations. About what you kind of intend to do (without letting too much of the surprise get out) and that it IS OK to use the Safe-word if necessary.

I have found, that when you have talked and you know each other's bodies and minds SO well, the use of the safe-word is rarely necessary. But, it should be used if either is uncomfortable. My Man has occasionally called a halt to things, mostly to see if I'm Ok, but it's OK to stop and take stock.

Y'all might need to do more talking first, so that there are no crossed wires.

I think P'Gell hit the nail on the head. Communication especially during rough sex or any BDSM related activity is very important, I wouldn't suggest waiting for him to call safe but have them acknowledge it's okay to use the safe word if need be. Another thing I noticed while watching a top friend of mine play with a someone who was submitting for the first time; some people hit subspace faster then others especially if it's a new sensation some of us get lost in the moment and forget to call safe when good pain goes bad. I would suggest starting out slowly and keeping tabs on them while you play; asking occasionally if they're okay, how they feel and whether their ouchies are still good ouchies it's a very good topic you've brought up.

Have fun and be safe

Besides, even now, when my girl and I have been together for a while, we still start off slow. It builds up the anticipation of what's to come. I believe it even feels more sensual that way.