Hi phoenix,
Don't worry too much about mix for grow boxes. The larger the container the less critical it is to have good mix. I'd just use a premium potting mix with added fert for this. callistemon are not fussy about fertiliser so any standard fert will suit it.
Callistemon will shoot from anywhere so not really critical where you cut it -it will change so much as it grows that there is probably no point in trying to predict a shape or style at this stage. I'd cut quite low. the aim at tis stage is to grow a trunk with good taper so cut low and grow tall, repeat as often as possible.
Usually easy to grow as cuttings so well worth trying.
Good luck
