One flag may send it to the pool, but it will be at the very bottom of the list. Then again, we probably won't even let it show up on the tool for moderators until 10+ users have flagged something. No sense boring the mods by sifting through tons of one-flag comments.
In the case of private user-to-user messages, of course, it would only take one flag. That flag system will grant us permission to view the message thread to keep everything in context while we make a decision on how to take action.