A) Within this scene I do not think a safe word is necessary; as we are playing by mutual respect, consent, for fun and enjoyment not in a BDSM environment where such words are used due to the sometimes dangerous nature of play! 
I also believe that a Top should be aware of how hard they are and paying attention to the bottom in front of them, literally.  In my capacity as a Top I am always aware of how the bottom looks, i.e. marks appearing, bruising, lumping, reactions, actions thus if I feel it necessary I will slow down the play or stop it completely. If you feel you want a safeword, this is not a problem and no-one will think any less of you for doing so, it is very good of you to ask and this should be respected by all players.
If a bottom/sub, any player asks you to stop then that is what happens, at the end of the day you have not signed over total control just consented to doing what you mutually enjoy. Just remember that if someone says NO within play and the administrator carries on that is ABUSE and as such illegal!
Also remember as a Top you need to ensure your own safety when playing with someone for the first time!
With this question in mind please DO NOT ask for it severe or hard if you are not aware of the pain this will cause, it is extremely dangerous to put yourself in that position with anyone and more so if you are playing with someone new or you are new to the whole play scene.