I disagree with your premise, but I am happy for you to set it up how you see fit. I agree that a few differences is a good thingThe reason I suggest the 5-7-5 split is due to weight of numbers.
There are a mountain of C listed players that get games.
With a 6-5-6 split there are a few teams that might struggle to find a 6th back or 6th forward who deserves a game.
Just my suggestion to keep this league playing a little different to the other one is all