Para: Can sims learn the Medium skill without living in a haunted house?

As has been said, you can build your medium skill without living in a haunted house.

However, you can't summon or meet Guidry unless you spend at least two nights in a haunted house, as he's not spawned until the second 9pm you're in said haunted house. I think, however, once you've done that, you can change the lot type back to normal and then summon him with the circle (once you've got your paranormal investigator's license.)