Questions on feeding my horse? PLEASE HELP!?

I have a question about feeding my horse grain. I have a 7 year Thoroughbred mare who im wanting to start up on grain. I am having a very hard time figuring out what to give her when it comes do grain. What is this best grain out there? Is mixing different grains better, like mixing sweet feed, flax seed, and stuff like that. if mixing grain is better what is a good grain concoction or mixture? How much grain do I feed her? I also want to put some oils in her grain, so which is the best for her coat and muscles? How much grain and hay do i feed her? She weighs about 900 pounds during the summer and about 1200 during the winter. Now this summer she is going to be out in a large pasture grazing. Should i feed her hay if she's going to be grazing all the time?? Or should i just let her graze and give her grain in the morning and night? Or should I let her graze, give her hay, and give her grain? I really appreciate it! Thank u and god bless.