That being said, at least in FL, you can have in the lease application that all adult occupants must be on the application.

Adult kids with limited funds vacationing with Grandma and Granoa who are footing the bills.  

I read some post on BP where they were suggested to charge more rent for 3 adults, but if that's not okay I'll not do that . 

I had a case where a resident had been severely attacked by a dog as a young person, and as an adult was VERY panicky whenever surprised by a dog.

If the tenant feels threatened by a roommate, they should work it out like adults or report it to the police.

The secret life of bees: Children put things in sinks and toilets without adults noticing.

I want to know what the healthy line of mediation is, and basically I want to remind them that they're in leases so they have to be adults.

For example, I mention that every adult must pay a $30 application fee, pass my credit/criminal background, make a combined income of 3x the rent, and whether or not the property accepts pets.

For example, I mention that every adult must pay a $30 application fee, pass my credit/criminal background, make a combined income of 3x the rent, and whether or not the property accepts pets.

This is time for adult lessons, which includes meeting your financial obligations.