Neither minis or goats *really* need grain, but if you do feed it, you'll be feeding a 1/4lb or so per day to the mini, maybe even less, like handful. Literally. That would make a 50 lb bag of feed last a LONG LONG time, about 200 days = almost 7 months. I would recommend feeding the mini either a ration balancer or one of the "mini" feeds made by a few different manufacturers. I've seen them at TSC... Since it lasts so long, it can be economical to buy the "good stuff" vs. the crappy feed.
I am not sure where you are located in KS, but if you are near Wichita, Topeka, KC, or maybe even Hays, you should have a pretty good variety of feeds nearby. Check out Atwoods, Orscheln, TSC, and every privately owned feed/tack store. You can also look online at company websites, they will list locations that sell their feeds. I am a big fan of Triple Crown feeds, and Progressive also has awesome ration balancers formulated for what type of hay/pasture you feed.
The minis at the therapeutic center I work at get a HANDFUL of Blue Seal Carb Guard 2x daily, along with a ration balancer called Sunshine Plus:
http://blueseal.com/product/?id=61&species=Horse&search=true They are also fed local orchard hay by weight, not free choice. They are all worked regularly in the lesson program, all year round, and they maintain their condition beautifully. The smallest guy isn't ridden, but he pulls a cart instead.
The goat is the same way - it really won't need any feed, but you will need to make sure that it doesn't push the mini off her feed. Seperating them will be necessary if she isn't pushy enough to keep the mini out. You won't need a full-size goat, you can get a small breed goat!