if there is more than one gear available on the bike its not really a singlespeed, by definition. even if you have to shift by stopping and moving the chain over with your hand.

You don't have to apologize for asking questions, but I'll have to agree that there's not a lot of advantage to setting up a bike like this. people build singlespeeds for weight and simplicity. cassettes are heavy, and a derailler is one more thing to maintain that can break. avoid using a chain-tensioner at all if you can, your drivetrain will be smoother and more efficient.