As she is growing still I would supplement to make sure I give my youngster a complete pellet and a small amount of Dr. Reeds. Unless you do testing you don't know what she is getting, and also what she needs is also dependent on her work load as well.
Only 4 flakes of hay & only 6 hours out doesn't sound great to me. I'd be looking at somewhere with full turnout, that doesn't keep horses cooped up as a matter of course, and allows free choice hay when grazing isn't adequate.
While I agree that *IF* a horse's diet is good, they shouldn't(generally) need a nutritional supp, from what I've learned, I would imagine it's an extremely uncommon occurence for especially a domestic horse's diet to be that good. They are generally imbalanced/deficient in a range of nutrients. Also you're talking about a rescue &/or growing horse, which are further considerations re appropriate supplements & diet. Feedxl.com is one awesome resource for working all that out.