I agree with what a couple of other posters have replied. With your horse being a mustang I wouldn't suggest grain unless he/she has a dietary need for it. What you need to know is was this horse captured from a wild herd or was it bred in captivity? Was he/she fed grain before and how did he/she respond to it?
You see it's VERY important to remember that mustangs are not accustomed to our processed foods and usually do quite well on the sparse, low nutrient containing forage they eat in the wild. If he/she was captured from the wild then he/she would probably not be used to eating grain... and even if they are/were feeding them grain I wouldn't suggest it unless there's a reason.
Like the other posters also stated it would be best to get the advice of a vet that you trust or anyone else that you might know that has quite a bit of knowledge in horse's nutritional needs.
As far as the round bale goes I wouldn't buy it either for just one horse... for the same reasons already mentioned.
Personally, I would stick with plain old grass hay in bales. That way you're ensured a low risk of either foundering or causing the horse to colic.
Good luck with your new baby!