You are running a business, not a charity.

Thank you for your service, one of the charities that NARPM supports here in the Houston area is Camp Hope, what an amazing group of people.

Unless you want to be a charity, run it like a business. 

I had a family who were schemers-always had stories and getting different charities to pay their rent and I was just tired of dealing with them so I gave them the proper notice at the end of their lease and in that notice I included the move out procedure and a reminder that they would owe $1600/month rent if they were still there on the 1st per section X of the lease they signed.

This is a business, first and foremost, not a charity. 

A local charity wants to rent my SFH in order to house up to 3 patients along with a 24 hr staff. 

I told my tenant that the contract is between us and as long as she pays rent on time through charity or on her own, I don't mind but I do not want to be involved with a third party.

There are calculators on this website, and more information on the website about how to do this mathematically.This is not a charity game .

The main house came vacant and a charity wanted to rent it for a good cause.

If they pay late, but always pay the late fees, they are usually OK to hold onto until their lease expires, then get rid of them.Your life will be better if you run like a business instead of a charity.