I haven't done this specifically because of the issues you mention, but...
Yes, with MCCC and other mods, you can have a household larger than 8 Sims, but CAS is limited to 8 Sims and just removes your extra Sims, like straight-up deletes them. What you need to do is use MC CAS to edit your Sims when they age up, or you want to change their outfits.
Also, split and move household is glitched out with more than 8 Sims. What you probably want to do is shift-click to remove from household, and then go into manage households and put them where you want them (in a household together with the other person you shift-clicked to remove from household, for instance). I would be nervous doing this with a child, even though I think people have done this on YouTube LOL, so I would save first if one of the people you were doing this with was younger than teen.
In fact, saving first is always a good practice when you're doing something unsupported. Save early and often! Good luck!
You need to turn on testing cheats and then click the Sim in question. "Remove from Household" should be an option. That removes them from your household and puts them in a "not currently in world" single person household, which you can then move to a new lot later.
Editing their outfits, click the Sim and go to MCCC -> Sim Commands -> Modify in CAS, which gives you just that one Sim.
