Personally would go...
Ben - Come Together. Would love to see him do Hammer To Fall, though. Dirty Diana would also actually be a good choice.
Fleur - Black or White (Due to the rapping).
Stevi - Fat Bottomed Girls would be a dream.

or The Invisible Man rofl.
Jay - Who Wants To Live Forever (His type of slow paced song).
Only The Young - We Will Rock You.
Andrea - Yeah, You Are Not Alone would suit him. Would be able to hit the high note(s).
Stereo Kicks - Radio Ga Ga sounds alright for them. Just let the black haired kid sing throughout.
Lauren - Somebody To Love.
Paul - (Whatever). Laugh if he did actually do Another One Bites The Dust, then went home.