Here's a suggestion for doing Survivor with alliances:

5 players or less join together on a tribe.  We play as usual only we vote out other members of our tribe before alliance members.  If the tribe goes down to the 5 remaining alliance member, votes are determined by the previous challenges score.  The player that did the worst from the previous days challenge is voted out.  The only exception to this is if that player wins immunity, in which the lowest player is voted out instead.

Thoughts?