How do you mix rotational and legacy playstyles?

I don't have a legacy family per se, and I have a lot of families I play between. I play with long lifespans, and aging off (for now). My legacy goal is to mix all my families in one way to the main family I play as, and alternate how many kids are born each generation. If the world becomes too cramped the non-heir kids will only have one child, with the heirs having at least 3, and aging will be back on. Then the next generation will have more kids.
On one save file, I only have three families I rotate between so far, so they are more in the legacy family mode, and I plan to get them as far as I can in generations, and only add a new family to mix with them as time goes on.