We use belts in our Drive Train and use exact center to center distances. That allows us to skip the tensioner all together. Although in the real world exact C to C is not recommended it is not a problem for FRC robots. We have been using this style of drive train for the last three seasons and we have never had a belt fail or slip. I highly recommend it to all teams.
If you are going to use belts, I would suggest using a 5mm HDT belt that is 15mm wide. We typically use 24 tooth pulleys, you might be able to go a bit smaller, but I cannot say that we have tried it. The WCP center to center calculator works excellent.
http://www.wcproducts.net/how-to-belts/
Before you machine any parts, order your belts and pulleys. If they are out of stock you will be kicking yourself in the end, we know from experience.