The thing to remember about the interaction of regeneration and fast healing is that they effectively give you Regen [R+F], where R is your regeneration rate, and F is your fast healing rate. For example and Shadow Creature Troll has Regen 5 and Fast Healing 2. He effectively has regeneration 7 until no nonlethal damage remains, at which point the regeneration stops, and any fire or acid damage is healed by the fast healing.
Just FYI.