I also didn't own the school, but I can tell you how it worked in a previous dojo:
Being an instructor meant that you were at least a shodan, had been attending the teaching seminars, and passed a comprehensive exam. Those who met all of the requirements, and with the chief instructor's approval, became ko-sensei. You would teach classes as the lead instructor, all on your own, with no direct intervention from the other instructors. Of course, before and after classes were open for such intervention, but never during.
Those who were at least nidan, had demonstrated solid instruction credentials, and had been ko-sensei for a while were eligible to become sensei with the chief instructor's approval.
Basically, the sensei and ko-sensei were paid for each class that they taught as the lead instructor of the class. You were still expected to pay your tuition, but that would be taken out of your monthly paycheck.
Those who were brown belt or higher, and were assisting the sensei / ko-sensei teaching the class, weren't paid for it, but at the same time, it would certainly count for something. If they were serious about becoming instructors, they would also attend the instructor seminars.
Once in a while, we'd reward them with a gift certificate, or give them a piece of equipment that they wanted for free, such as a new set of sparring pads, or whatever was appropriate for them. Their help was always appreciated, since sometimes, you might have had a large sized class, and had to give first day lessons, or you would let them give the first day lessons to the new folks. The least one could do is show that appreciation once in a while.