If you are talking about the Lucerne Farms Hi-Fiber chopped hay (they are working with Manna Pro on distributing their product, Manna Pro name is on the bag), then yes, you could feed it to your mini. I'm feeding my senior stallion their tim/alf mix (molasses), and my Cushings horses get their Hi-Fiber Gold (no molasses); just started this month, and so far, its going well. [i don't normally feed bagged hay products, but I had to do something for these 3 horses to get weight on them; our homegrown hay wasn't doing it for their special needs. Its just a portion of their daily diet.]
If that is all you are going to feed for a hay product, then you'll need to feed 1.5-2% of his bodyweight daily (so if he weighs 250#, you'd need to feed 3.75-5# daily of the forage). Personally, if it were me, I'd mix it with regular long stem hay, so he wouldn't get quite so much sugar (molasses) added to his diet; in this case he'd only need a pound or so a day to supplement his baled hay. [i know some people feed exclusively one of the bagged forages, but in the long run, baled hay is usually cheaper.]
Do you have any horse experience? Especially full time care and feeding? If so, feeding and caring for minis is mostly like caring for their full-size cousins with just a few differences. The biggest difference being how much they eat; coming from full-size horses, I still have a hard time sizing down the minis meals, going from feeding 15-20# (or more) of hay daily down to 3-6# daily is quite the change (I have bigger minis, so feed about a 1/3 to them what I fed to the full-size). They can do quite well on their hay/forage plus a vit/min supplement. Although, some do need more feed or special feeds (growth, performance, etc).