You are missing my main point here. In large because I did a poor job making it, so I can't even remotely be upset. :)
Insurance works by getting people to pay in to it that will not get paid out of it. Pretty much by definition. This is why we pay insurance for cars, instead of just avoiding cars. Same for title insurance. Same for life insurance. The vast majority of people that pay into these schemes do not get paid out of them.
Now, the only way you can use a pool of "healthy" people to get low costs, is to have a pool of healthy people that are not using medical procedures. You can do this by discriminating out people that need procedures, but realize this also means you discriminate out people as soon as they start to need these procedures.
Car insurance kind of works here. Many will take on "higher risk" kids for insurance. But as soon as those kids make a claim, they will refuse to re-insure them. Same with houses. Make a claim, find that you have a chance of needing to get new insurance policy.
So, for your little hypothetical. My point is take the 50 that are healthy, and have them pay lower rates. And then, as soon as one of them hits a high cost procedure, find that you don't have enough to cover it. (Just one cancer patient will blow out a pool of 50 people in a heartbeat.)
Could it work to keep costs down? Yes, but only in a way in which would not benefit you. As soon as you needed what those costs were supposed to cover, you would be kicked out of the pool.
This is like the poor sods that think they can do a health savings plan. Only if you are very wealthy, and still only if you never actually get into an accident or need it.