Every horse owner has a different feed program of what works for them; so, you're going to get a lot of answers.
I live in the middle of nowhere and have limited feeds available to me locally, but luckily I can get a few Progressive Nutrition feeds, I love there ration balancer; local store just switched to a new senior feed (so I'm in the process of changing my senior gelding over - its more economical than the Progressive, and he likes it); I special order beet pulp; and I can get oats. [We raise most of our own hay (we have cattle), but I still buy some grass hay for the horses.] This fall I drove 5 hours one-way to pick up some grass hay pellets and I'm looking into a closer store to see if they can get in the grass hay pellets for a reasonable price.
Anyway, my minis are all on Progressive Nutrition grass balancer and grass hay; all my minis are B-size and 325-400#. Maintenance level is 1/2# ration balancer, pregnant mares get 3/4#, lactating mares get 1-2# (can't remember the level), growing horses get 1-2# (depends on age and size). Most are on free choice grass hay and all get 1/3# beet pulp (dry measure, then soaked, about 4 cups soaked each) in the winter. [i do have one A-sized stallion, he weighs about 180#, he gets 1/4# ration balancer, 2 cups soaked beet pulp, soaked t/A cubes (4-5 cubes), and some oats (haven't measured his oats, as his program is being adjusted) plus free choice grass hay.] One skinny gelding is getting a little bit of senior for extra calories, he's finally gaining, so won't need the senior much longer (never had an issue with sand til this guy, so put him on a sand product for a week, and he started gaining). My horses look pretty good on this program, and would look even better if I could get better hay (it was a bad hay year, so we are short on hay, I'm looking to find some more to buy, if I can - if not, I'll feed some hay pellets to stretch the hay).