I'd say do the Muy Thai first, then get into BJJ if you aren't planning on doing MMA. Or vice versa. While its possible to learn both at the same time, you are short changing yourself IMHO. Learn one well, then learn the other well - you have your whole life to learn both.
As far as meditation goes, I suggest a book called Zen Training by Katsuki Sekida
. Took my zazen to a whole new level.