Utterly Mad

Given their numbers and abilities, it'd probably be best if they remain NPC only, at least for now.

I'll add in something along the lines of "They cannot read minds through vacuums or through completely sealed objects, as it is a biological function which requires direct access to the brain."

That should help to limit their abilities a little bit.

I can definitely understand not wanting a single player to ruin the story for everyone, especially with such a small playerbase, but I would warn against restricting any attempts at interfering with someone else's schemes or plots or the like: If people don't feel like they can ever make a difference, then they may not try at all.