You don't need a tensioner hanging down with an Alfine on a vertical dropout. Just get one of the eccentric bottom bracket kits being used by the single speed crowd. You may also need a halflink depending on your sprocket sizes.
Look here for them
and here for how to calculate number of links required
If in doubt talk to a single speeder.
If you are running on a full sus like the OP asked you will need a tensioner