Ninjitsu does take effect, it is azure fang's ninjitsu. Resistance also take effect.

Poison is capped at 1999 damage per stack, up to 4 stacks can be applied.

There is no percentage life limit. If can kill a 7k max hp ninja from a much weaker player instantly with poison if you can somehow put 4 stacks on it without it taking life damage. (For example, the opponent spam shields and non-debuff clearing heals on the ninja.)

Under normal cir*stances, you will not reach 1999 damage per stack. You will only reach it when you are fighting someone way weaker than you.