I only do 3 gallon batches and usually have about 3.25-3.5 gallons fermenting in a 5 gallon carboy. I've never had to use a blow off tube, but if you are going to be using 3 gallon carboy for 2.5, they there is a good chance you'll need a blow off tube. Just cut all your ingredients in half except for your yeast. I don't change my yeast and in most of the time I don't need to make a starter w/ any liquid yeast. I use Beersmith, which has a scale button, to change a 5 gallon recipe to a 3 gallon one. As for kegging, the only issue you'll have is that it will just take a little more gas to fill up the extra space.