Rules are general, each horse has specific reactions. The rider must adapt the rules to fit the specific horse, then teach the horse which movements are actual cues, and which are just the horse/rider equivalent of “white noise”.
Also, balance must have at least as much to do with what the rider’s upper body is doing, and with the rider having his/her upper body centered and balanced over the rider’s own center of gravity and over the horse’s center of gravity. If the rider is correctly balanced, he/she can stay in place and balanced with or without stirrups, and without tensing the leg muscles or applying pressure to the horse’s sides.
If you have not already read it, you might look for a copy of Sally Swift’s book “Centered Riding”.