I've got six nannostomus eques (aka tubemouth pencilfish), and they are completely peaceful. They hang in the water at the surface most of the time, doing their best imitation of sticks. You'll read occasional stories of beckfordi pencilfish going rogue and terrorizing other fish, but those seem to be very much the exception. If you can find (and afford) Espei pencilfish, well, that would be cool. Their stripes are vertical, so they look like they've been grilled. My eques only look grilled at night.