3 point mounted disc mowers

A couple questions about these type of disc mowers that just mow and do not condition, I am thinking generally speaking the rotors are all the same size. Is there any rule of thumb as to horsepower requirements per rotor? That is, if I am looking at a 10 rotor mower, should I have a 60 PTO hp tractor if 6 hp is the number? Assuming you have a good curtain installed properly, are these mowers dangerous with open station tractors? Do you mow any differently with these mowers than a sickle mower? That is, I generally cut my field out with 5 or 6 outside passes, then start sectioning off lands. Do you do anything differently with a disc mower? Are there other difference (besides installing a seat belt on your tractor as you will be cutting twice as fast.....?).

3 point mounted disc mowers