I like the idea of the setup, you'll just have to make sure there aren't any areas where lower ranking horses could get trapped by a bully or more dominate horse in the PP. How wide do you plan to make the PP and the lanes leading to the barn? They need to be wide enough to allow passing by the horses.
There is also the concern about the footing in the PP with that many horses on it all day, and possibly all night. They'll tear the ground up pretty quickly, especially if it's muddy out. You might need a sacrifice area to contain them for a while when it gets really bad out. (I like gravel sacrifice areas better than sand, drain better and wear the hooves down more.)
Also, will you have one or multiple watering stations in the enclosure for the horses? Or will you keep water in their stalls?
It'll be a bit of a learning experience at first for the horses at meal times, you might want help to guide the horses into the stalls until they get the routine down pat. Keeping in mind that any new horses will have to learn this routine also, but shouldn't be too big of an issue.
As for the paddocks, do they have shelters or would the horses be stalled at night?
For the owners mixing their own supplements etc, I like to do that myself anyways, but some owners might not have time to do it. You might be able to charge a small fee to mix the supplements yourself or have a menu of boarding options available for the boarders to choose from, each with a fee.
Or you could offer a horse crazy kid the opportunity to learn about horse care by feeding etc, in exchange for riding time or some other reward.