Who told you to not use a tensioner? What was their reasoning?

Your cog and chainring are a set distance from eachother, no room to move. You could alter the amount of chian you need by chaing the gearing around. And if you are lucky, and with a lot of work, you could get something that will work. Or you could use a high quality tension(Surley makes a nice one for $25) and be able to use whatever gearing you like, and know that you will always be set.