ye, the yellowing leaves are most likely N defficiency. You should use a fert with higher N than P and probably K. Don't use too much in an attempt to speed up growth. If the plants aren't in too big pots then transfer to different soil and bigger pots as generally miracle grow is shit. The fact that it is short and thick is not a bad thing but 5 inches seems pretty tiny for that age, don't raise the lights, that wouldn't be the problem