Congrats! I have an "Annie", too...but she's a TWH. However, I just got an appendix quarter recently myself. I give her different feed than Annie (the walker), because Annie is very thin and has a higher metabolism, plus she's still growing.
How old is your mare? My quarter (her name is Belle) is 10 years old and can literally stay sleek and muscular on just grass, so we have her on pasture, plus about 10 lbs. of alfalfa as a supplement because the pasture is poor in some areas. I like to feed a higher fiber/roughage diet as much as possible, so I give her Safechoice for her grain. Just 2-3 lbs/day. She does great on this with light riding. I think a lot of people will tell you that you don't even need to feed grain, and I agree with this. I just give the Safechoice to round out her vitamins/minerals.
There are so many diet options, and talking to your vet is a good idea. PeggySue (one of the users on this forum) happens to be a feed wizard and can tell you pretty much anything. :o)
And...we want pictures of your new horse!! :)