Just think carefully about the number of ponies on each field. It can soon be over grazed, and if you have other people's ponies on too, it would be unfair to be in a situation with no grass, so they have to go. Personally, I think the number of ponies you have suggested is too many for the acreage. At the moment you will see a lot of grass, but from my own experience, horses can eat an awful lot oper day. I had two ponies and one horse on about 2 acre paddock recently, and within a week they had bared it off.
Taking on liveries - insurance can be an issue, as in public liability etc. and if a livery owner got kicked by someone else's horse for example, so all need insurance, including you for public liability. I know here , if you take on more than three liveries, you have to pay business rates. OK to do it, but there is always some person who thinks it not fair, and would report you for "running a business"
You would need to poo pick the fields daily, to ensure there is enough grass, and make sure all the horses are wormed before coming on, and then together, to ensure you do not have a worm burden.
With careful management of the land, it does work. But, ideally you do need a paddock for using, and then one for putting them on while the other is resting.