While Safechoice and grass hay may be good for two grown horses that have gotten proper nutrition all their life, it's not a good choice for a growing underweight filly whose been malnourished her entire life. It's like playing catch up with her, which is why she needs a different diet. :)
ETA: Alfalfa is also a good source of protein and calcium, which is also good for a growing horse. Though it should always be used in moderation, because too much of either can have negative effects.