I'd make a photocopy of the student's driver's license or passport, and I told them that this is a contract between me and them and if they break the contract, it would be them getting sued, that they were responsible as an adult with no parents involved. 

Living with friends here and there.Applicant 4: suspicious and issues already...supposed to be 5 adults 2 kids...only 4 applied...the daughters boyfriend the parents told me about living with them is suspiciously missing as an applicant.

Tenants are adults, not children. 

How many people (adults & children) will be renting this suite?

If vacancy is high because your metro overbuilt a little, some young adults delaying their entry into housing, or increased roommate activity then this facilitating of A <- B <- C will be met by lower rent and eventually (and friction makes this a slower multiyear process) the vacancy will fall most of all on owners who aren't motivated enough to cut rent to avoid some vacancy, but also will increasingly get shifted down to the worst C properties (which makes sense when you think of where long term vacancy ends up in metro's with higher overall vacancy rates... its always in the absolute worst neighborhoods). 

I worked with them to get moved out amicably.After that, I had decided that my criteria included any adult had to have a certain minimum credit score. 

I do feel like anyone who would rent to this person needs to know that they simply do not know how to clean and live like an adult at nearly 30 years old.

Many household products are labeled and marketed as DISPOSABLE and/or FLUSHABLE; many baby and adult personal hygiene products, along with household wipes and cleaning towelettes are labeled both disposable and flushable.

Most adults don't like being told by other adults how to live their lives, manage their finances, drive their car, raise their kids, etc. 

And, while that might have been worth discussing had they approached you as "adults," they chose to financially harm you.