When you start taking people tubing, keep in mind that riding in a tube is a lot like riding in a small boat. Easy at slow speed. Once you go fast enough, the tube gets on plane. Going in between can make a big wake in the tube. According to the passengers I have dragged around the lake, this in between speed is not very enjoyable. So while you may not want to quickly transition from slow to medium, your passengers may not enjoy the learning curve in between.
It will be much easier for you to get used to pulling with a lighter passenger as they get on plane at a lower speed. We transitioned to a big big tube as it is easier to keep my big kids on plane and it is very rare that they tip over no matter how crazy they behave. (Or I misjudge the sling shot maneuvers)
Early on I would have trouble with finding a speed that the tube just got on plane and the boat did not go too fast once it got on plane. It always seemed like I was going too fast or too slow. There was little difference in the boat rpm. Once they came out, the boat would come out and I would try to slow down and the boat would drop back in and the tube would drop back in.
My life got a lot easier when I realized that if I tilted the motor up a bit to make it a little less efficient and lifted the bow up a bit, I could achieve a speed where they were on plane and I was not. Different weight riders might take slightly different trim amounts to reach the balance.