[@ihinka]
Engage with them, Converse with them, anything involving your PC (so long as you're obeying the rule of keeping NPCs from being in several places at once) is all fine.

If you have reason enough to use them outside of your scene, away from your PC, I think you'll be okay to use them. Just be careful to stay to their character.

I'd shy away from using Blacklight, Marching House, and Nightmare though.

Though really, what did you have in mind?