I definitely think that playing multiple instruments is beneficial...I've been playing piano since I was 4 and violin since I was 10 and I think they've really helped with guitar...I've been teaching myself the past 6 months or so and am probably farther ahead than if I didn't play anything, just for things like music theory, rhythm, sight reading and even a bit of the mechanics of playing...
40ftsmurf your house sounds like mine...we have three guitars (two acoustic and one electric that I just bought), piano, keyboard, bass, drums, and my violin. No mandolins though...I'm thinking of buying a melodica next :)
