I would absolutely get an application filled out for all adults and run the appropriate background checks. 

If they are responsible adults that I determine are worthy of renting my apartment I should be able to trust them to take care of a gun safely as well.

You could put teens or non-couple adults in that configuration. 

E-4 and below, typically very inexperienced as a Tenant, and frequently as an Adult, with all that goes along with that stage of life.

However, am I legally responsible for their behaviour as they are all adults. 

Hi guys,One of my tenants wants to move her adult daughter with her.

This of course only applies if you are dealing with normal responsible adults, with jobs.Small claims court is inexpensive and designed for th eindividual to manage themselves.

I am not where I want to be but I am further than I imagined as a kid or young adult.

I think most adults prefer the elongated bowls but I'm concerned about installing a tall toilet for a rental.

The image that you have of some hippie bum with no job simply doesn't match the pot-smoking population at large - responsible adults don't let you know that they smoke weed if they don't want you to know.