As NorthDakota said, I also have the MCCC mod. However, while I do have kids move out and live their own lives, I do check in on them to see how they are doing. I do not interfere unless I have to for some reason.
You can pick an heir a few different ways. You can go with the traditional "first born" selection, you can go with the "first boy" selection, you can do with democracy, have other people cast a vote for the heir, you can have two sims fight (called strength) to be the heir method, or you could do a completely random roll for the heir. It is situational. Depends on what you want to see from your legacy gameplay.
I'm doing a decades challenge, where you start in the 1890s and work your way up to the 2010s. I'm in 1910s right now, so naturally, the heir is the first born boy given those times. It will change as I get into more modern times. So it really depends on how you want to play your legacy, and there's really no wrong way to do it. The legacy challenge website lists all the various ways you can choose an heir!
