I would use the 15 gal for 5 gal boils, and the 20 for 10 gal boils.
In terms of the pickup tube for the kettle, I just have a piece of tubing that comes out of the back of the ball valve, does a hard 90 and rests up against the side of the pot. My whirlpool port sits about 3" off the bottom of the pot, and is just another ball valve on the outside, with a 90* elbow on the inside, that has a 45* tube out of it, that pretty much repeats the diameter of the pot. It can be used for both 5 and 10 gal batches. Nothing pretty. But it gets the job done.