Perhaps things are different here, IDK. In the past I have been obliged to 'share' grazing with several other horses in a large area, and you don't always get to pick who they share with.
On these occasions it has been grazing only. No barn, tackroom, hay or overseer. Each person was responsible for their own horse's care feeding etc. The landowner kept the watertroughs in order and fences, but often it was up to us (the boarders) to point out if there was a problem. TBH I recall hardly ever seeing the landowner unless I had to report issues or make payments.
If the paddocks (fields) are big enough horses lower in the pecking order will keep away from trouble. You can also help prevent trouble by spreading the hay out well so all animals will get some.
Life happens and there are times you can't get there every day and we would watch out for each others horses. Its easy enough to phone and let someone know if there is an issue. Ultimately it is a horse owners responsibility to check their horse, whatever else is arranged. A good idea is to keep a laminated sheet, book, pad or something with boarders phone numbers where they can each contact another should the need arise. Have a copy where boarders can easily refer to it.
You have to trust most people will be responsible and decent about everything but in the event someone isn't prepared to be fair you may need to tell them to move on for the good of the rest of the setup. It will be part of your responsibility to sort out niggles and you will have to be fair and firm about it.
If the horses need extra feeding it is up to the owner to do that or arrange for someone to do it for them (barnowner, fellow boarder, family). At some places you could pay a bit extra and get a separate paddock for your horses.
What I would do is decide how you want to set up your property. Have an area for the boarders to bring their horse to groom feed etc, separately from each other.
- What storage room will be available for them.
- What rules will you need (unfortunately, you will need rules)..for instance if boarders keep tack and feed there and it can't be locked away separately, you will have to insist No Borrowing without express permission.
- You are feeding hay. Decide how much, how often, stick to that strictly, then the boarders know where they are with other feeding.
- Tidying, expect them to clean up after themselves. Have a manure area and wheelbarrow/shovels available.
- Check out the legal aspects in your area, and a written contract between you and boarders will help prevent issues.
- Personally look at what other boarding places in your wider area are charging and what they are providing for the money. Decide on an amount and offer your wares. No responses could mean you want too much, but you will work out the details. If you think your charge is fair be patient, it might take a bit of time to get known. If you charge too little you will find other things you wont be satisfied that your time has worth.
- You could arrange with a good farrier for regular visits to work on several horses for a small discount each. Get a general idea what sort of time suits most boarders then it is up to them to work in with things. I'm sure you can work out some details.
You don't want to set yourself up to be changing things at every whim, so you need to decide what will work for you. Especially to start you will need to be a bit flexible, and when something doesn't work discuss it and go from there.
The one thing I cant really account for is your weather. Our winters get nowhere near a cold as yours (except the high country) so you will need to consider if you need to address that as an issue.