Genes play the biggest role here, along with your previous athletic career.
If you're a twig, focus on mass building, at least for now.
If you can cope with lots of sports because you enjoyed an extensive athletics education in your youth, that's a big bonus.
If I understand correctly, you priority IS bodybuilding, right?
if you have trouble gaining mass, it's probably foolish to do so much MA.
weight gainer shakes and calorie dense foods are a must.
a reduction (read: ditch them!) of isolation training and of exercises that cause irritation, inflammation etc is a good idea
BJJ is a good idea because you can play rel. hard without greater health risk like, for instance, with thai boxing. BJJ is also great because it focuses on mobility and fluidity, therefore complements BB very well, which lacks these qualities and can make athlets stiff and awkward.
Thai Boxing + (real) bodybuilding would be nearly pointless, from my experience, without drugs or good genes with superb planning.