Isn't high fat ration balancer a bit of an oxymoron?
I thought I'd chime in and plug Triple Crown 30% Supplement, which is what I feed my easy keeper horse. It's the lowest NSC RB I've seen yet (9.8%) and is very cost effective; it costs about the same as SmartVites per day, at least for my 1000lb horse.
An Oxymoron is an incongruous or self contradictory statement. That doesn't apply. Many Ration Balancers are not high in fat (they are high in protein); however, the particular RB I use is high in fat (22% vs. 5% in others roughly) because it is made for horses that tend to be hard keepers or in heavy work.
OP, for a mini you can call Smartpak and get a smaller 'pak' that fits the nutritional needs of the smaller horse, but, honestly, after shipping, you will be spending a lot more than buying just a vitamin - if you are really confused about what he needs I would recommend a 1 month membership to FeedXL.com. You can mess around with different diets, even plug in soaking hay and put in that your horse has laminitis, and try to come up with something that will be cost effective and nutritionally effective for your horse. I did it for my 3 horses, and I cut out about $400 per year in costs just figuring out what I did and did not need. If you have a scale of some sort, it does help because then you can weigh your hay. A bathroom scale works fine because you can just weigh yourself, get off and pick up the hay and then get back on and do the math - that's what I did for a really long time until my husband found an old mailing scale at a garage sale, but I have now gotten very good at judging the weight of hay! I still weigh for my easy keepers.
I would look for local grass hay that is for horses in your area and not go with Johnson grass.