This article (which references this publication) suggests:
we expect people that are approximately the same size, and running at
approximately the same speed, to use approximately the same frequency.
That is an explanation for why the best marathon runners use the same
So, per your question about theoretical advantage of leg length, I don't think that's really the best question to ask. Rather, ask, "What is the best cadence for my leg length?" since we can't really do anything about our leg length (much to my chagrin).
I will suggest that in addition to working on gait/cadence, it's incredibly important to get correct form. One of the biggest inhibitors of efficient running is heel-striking. You lose an incredible amount of momentum when you heel strike. There are several types of shoes that are designed for different running forms. If you live in a decently sized city, most running stores will actually have a setup where you can run and they analyze your gait, degree of pronation, etc. for free. I'd highly recommend doing that for anyone interested in doing any type of running, whether recreational, competitive or otherwise.