I have 5 horses and free feed a round bale (1100 pounds+/-) of either coastal bermuda or wheat hay, which lasts a week. I have:
1 - a 23 year old gelding whom my daughter uses for parades and flagging events, and he gets equine senior, omolene 200, and a joint supplement.
2 - a pregnant 18 year old mare, and she gets mare and foal (or omolene 300) along with the hay.
3 - a 19 month old colt, who gets the mare and foal as well as the hay, since he is still growing.
4 - a 8 year old gelding and 10 year old mare who primarily stay on hay or the omolene 200 (not always fed this daily).
If/ when they start dropping weight, I will put them on soaked beet pulp with a little canola oil in addition to everything else. There are varied responses to what to feed for weight, but I found this works best for my guys.
As for what to do with the filly... as it sounds like you are planning on keeping her, then work with her with the intention of her just being a pleasure riding horse for now, and see what she develops into. Certain competitions may require sending her off to a specialized trainer, and that could get very costly. I do love her lines as well, and you should have a great horse in both the mare and filly both.