[Reyoki]

You should totally play P4. I'm still working on getting the best ending on it, myself.

Well, since they're entering a Mirror World, I don't think mirrors would be the best idea. In P4 they enter the TV World and use cards to summon, and we're entering the Mirror World. Since the logistics of first gaining access to the Mirror World involves a candle, candles are probably the best way to go for summoning.

Once a character gains a Persona, the persona will indeed appear when the person looks in a mirror... but only for the person who summons it, and only as like a transparent image behind the person. Kind of like they're being haunted by a ghost that only they can see. That's the basics of how the rumor mentioned in the first post works the way it does.