My Assassin is level 11 (almost 12) and I just completed doing each of the quests for secondary jobs ... Just to keep my options open.

What secondary would you recommend for leveling a PVE assassin? I enjoy solo play with few henchman, but also would like to group once I've honed my skills and am not a complete newb anymore.
So....which is it? What compliments 'Sins the best?
I was thinking Monk to remove status (like blind) or Mesmer to break spell casting. I also thought Warrior abilities might compliment me ... But I'm not so sure.
(Also, the "Master" NPC told me that once I've chosen a secondary I can't change it, but this is contrary to what I've read on the forums ... Which is it?
