Yeah, you have to build up poison in NPCs. In that building with the two demons right before the archers, if you stand back far enough, their arrows won't reach you.
Siegmeyer escapes the boulder trap since they change which way they roll when you enter certain rooms. That or just kill the Giant that sends the boulders down to the trap. Not sure if you have to kill the Golem to get him to go to Anor Londo, though