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.

there's an inherent difference between messing up someone else's plans and doing so in a way that's just feels blatantly unfair. If someone's character were to happen upon or seek out and find knowledge about someone else's plan and decided to mess with it I'd be willing to let that happen, I'm just not willling to give people the ability to say " I use my mind reading powers to find out what this person is planning" because, for 1) there's not story, there's no work, there's nothing interesting about that from any perspective and 2) it just feels completely unfair and annoying to have someone elses character know everything yours is thinking and being able to act on that knowledge.

It's not refusing to let players influence things it's a refusal to 1) not make them work to do so and 2) to do so in what feels like a completely unfair way to other players who don't have characters of a specific race because that race doesn't interest them.