This is going to be a very long post!
There are a LOT of choices for supplements and everyone is going to have a personal favorite/opinion about each. I personally feed orchard grass/timothy because it provides adaquete protein, calories, and carbs. I supplement my hay during the winter with 6 cups of soaked beet pulp, 3 cups STABALIZED rice bran pellets (I use triple-crown), and 1 to 2 cups of ground or soaked flax seed. Beet pulp is a wonderful supplement, as it is a high calorie forage for the horse, and not a grain. It is high in calcium and low in phosphorus (two minerals that must be balanced within the horse) so I feed it with the rice bran which is high in phosphorus and low in calcium. Triple crown rice bran is stabalized within minutes that the rice is "husked", for lack of a better word. This is extremely important as it prevents fermentation and allows for a better shelf life. Rice bran is high in fat (this particular supplement is 20% fat) but it is also super high in starch, so I know that with a horse your size it is best to stay at a pound or less a day. It is important if you feed rice bran that you feed something with extra calcium, so if not beet pulp then the alfalfa pellets would be acceptable as alfalfa has a lot of calcium. Lots of people dislike alfalfa, as it is a "hot" feed that is high in energy. For some horses, alfalfa is a magic cure that puts on weight and they live happily ever after. Arabians are hot blooded horses, and for many of them alfalfa is not a good option. I personally do not feed alfalfa as there are many other alternatives that are healthier. If you do choose to feed alfalfa, hay is better than pellets because the horse consumes it slower and they have less of a chance of founder. Flax seed is high in Omega 3, and is very tasty and palatable to the horse. I like it because it gives that extra fat supplement and makes the horse very shiny and is a great alternative to grains. Omega 3's are not found in grains and only small amounts are in pasture grass. This combination of food amounts to about 4 pounds of feed, and it is only a supplement to the best quality hay I can find. I pay top dollar for my hay and feed my 900 pound arabian gelding around 13 - 15 pounds a day. He is also blanketed when it is below 40 degrees, which also aids in keeping weight as less energy is used to keep warm. All in all, this adds up to an average of 18 pounds of food every day, plus a couple carrots and pieces of fresh fruit for varieties sake. Just remember that a horse should eat 2% of his total body weight per day, so an 800 pound horse should be eating 16 pounds per day of forage, or forage supplemented with grain.
So, my recommendation would be to feed something similar to what I am doing, but I can totally understand if you do not want to spend as much money each month as I do on feed. I only have one horse, and his monthly bill averages at 145 dollars for feed alone. If you want to, you can put your horse back on senior feed. I personally like TDI-Senior because it is cheap and contains beetpulp, and I think it is better quality than other brands. Sr. Feeds are complete feeds and almost all are grain based. These will not make your mare hot, but they will give her more calories and more energy. They are also very safe for novices to feed, as they are balanced for you and you just feed to your horses weight. I personally do not feed complete feeds, and I wouldn't do it unless I couldn't afford feeding the way I do now. Anyway, I'm done with my super long post and hope this helped!