Rather than trying to come up with a panacea, an understanding of the underlying issues will be more beneficial.
Why is a 6' launch rod better than a 1' launch rod? When would an 8' rod be better than a 6' rod? What are you accomplishing?
A rocket is stable when it has reached a sufficient velocity at which the relationship between the rocket's center of gravity and center of aerodynamic pressure comes to bear on the airframe.
For most model rocket purposes, that velocity tends to be about 48 feet per second (32 mph). So, before the rocket can be stable in the air, off the launch rod, it must have already achieved this velocity. A simulation program will use the motor's thrust curve and the size and weight of the rocket to determine how far the rocket needs to travel up the rod to reach that velocity. If that velocity has not been reached prior to the rocket's first lug clearing the rod, then there is a high likelihood that the rocket will be unstable, even if the CG/CP ratio is correct.
The force that produces the rocket's velocity is the thrust of the motor. A motor's thrust is not consistent across its burn time. Some motors produce high thrust at the beginning of the burn; others need time to build up thrust.
For the former, less time on the rod is needed for the rocket to reach 48 fps. For the latter, more time, i.e., a longer rod.
Each rocket and motor combination is going to have a different rod length requirement. For many, the difference is trivial. But, then why not just use a 20-foot rod so that you can be sure you have enough?
Because long rods bend in the wind, even steel ones. So if only 4.5' of rod is needed for a particular rocket/motor combination, that will give you more stability on a windy day than an 8' rod, which will bend more. A bending rod also slows down the rocket by applying friction to the lugs.
Thus, your statement "a 1/4”x6 ft Sainless Steel will work for MPR using F and G motor" is an oversimplification.
ThrustCurve.org is a nice starting place for understanding what rocket/motor/rod combinations work.
When you move up to rails, you get the added benefit that they do not bend in the wind, and so pretty much you can't have too long of a rail. I always go for the longest rail available to me. If I am heading for a shorter rail, I want to know if it is long enough. ThrustCurve, OpenRocket, and RockSim will tell you that. And two of them are available for free.
