I've found its better to expand organically than to make a million hyper specific breakouts when starting off or even migrating, but there's nothing stopping us from setting a Magazine up for it at anytime.
Typically it's best to start off with a few, active, places for people to be and then break off side areas if one topic is hogging too much of the spotlight. One or two active places is more attractive to potential new users checking a place out than to have a dozen places that all only have a post or two each every other day or so.
Just my experience though.