Hi folks! I'm trying to make a Sim wear a CC when a buff is active, some advice?

Not possible with MC4 last I heard, you need to manually add it with S4S. So make the base of the buff for whatever else it needs, then once that and the rest of the mod is completed export the mod, open in S4S, then add in the CC appearance modifier. You can use the TDESC and/or find an example (the vampire bite is one) to understand how the layout looks.