The problem with bagseed is you really have no idea how they are going to grow. If you buy seeds online you'll know exactly what their growth behavior will be like and you'll be able to compensate as needed. 20 inches is pretty small, but it's been done. Check the grow journal section for ideas...and as beyond305 suggested, topping, FIMing and/or LSTing will be something you'll want to look into.
Some bag seeds can easily be kept under this height, while some won't be so easy. That's the problem with bagseed, you don't know what you have.