Ah, college life!!
Coed buildings by floors should be ok. Really, it’s just like apartment living, right?
As for the dragon poster, as long as the poster does not have satanic/witchcraft elements or overtones, I would not be too concerned about it. The modern notion of a “dragon”, while probably developing from the Biblical correlation of Satan and a “dragon” (or “great serpent”), has now progressed beyond only Satanic connotations (see the success of Anne McCaffrey’s “Dragonriders of Pern” series, where dragons may look like dragons, but are completely different from the sterotypical “bad” dragon).
That being said, there is always the danger that one becomes attracted to dragons through these less-harmful notions of “nice” dragons, and then becomes more attracted to the dangerous, more Satanic notions of dragons.
So, pray for your rommate, and express concern if he goes beyond simply thinking it “cool” to have a dragon poster. Try to lead him by example yourself, and if opportunity arises, perhaps put a poster or two up yourself that has better meaning (a movie poster from the “Passion of the Christ” comes to mind) and might strike up a conversation or two.
Just my :twocents: