increasing the light schedule to 18/6 or 20/4 shouldn't hurt them. but if you're going to be flowering 7 plants under a 250 hps you're going to want to keep them small, as in you could probably start flowering now. it would be sort of 12/12 from seed yield (7-14 gr per plant) but might be the best option if all 7 are confirmed fems (you said you could identify them already?)
as for nutes, you just have to find what you like. i use humboldt, most ppl like fox farm, some botanicare. sure u can grow without them but using them for flowering will improve the quality of buds and your yield just be careful to introduce them to the girls slooooowly (never start with recommended dosage, 1/4 at most) overferting can kill your yield.