hey sull,
yes you can top a plant t make it more BUSHY.. and to produce more "main" colas..
But keeping a plant short requires training.. or VERY VERY early flowering.. BUT mostly, it requires TYING the plant down.. pulling branches down..
instead of topping, a good technique is to tie a string to the top of the main cola and then tie a screw or a bolt to the other end of the string... this will gently pull the plent over... WITHOUT breaking the branch..
how much vertical room are you working with?
iloveyou