Poison, Stoning Poison, and Deadly Poison, on merge automatically set their...

Poison, Stoning Poison, and Deadly Poison, on merge automatically set their max_power to 5x the power set by default.  This means that their damage cannot stack more than 5x times by default.

Poisonous Bite is an unarmed attack (prevents stacking of weapon bonuses with built in multiplier).