With 20hp on your small rib you should be fine towing.
We've been towing toys, kneeboards and wakeboards with teenagers/adults on and two up in our 3.5 and 3.8m sibs powered by a 20hp 4 stroke Suzuki. Full power required to quickly get out of the hole up onto the plane but then easily cruise on 2/3 throttle at around 16/18 mph.
Surprisingly the toys take more grunt to get on the plane than the kneeboard or wakeboard. Currently have two toys we use.... A donut ringo and a steerable 2 person tube. Out of these two the donut needs more grunt. With the tube we can pull two lightweight adults on a good day.
These are our two....the steerable Airhead Slash is a real hoot.
PS - Good vid Mat