I can think of a few buildings that are condos but no self respecting adult would live there.  

All adults did not complete their application so you can't proceed with considering their application.

You need to make sure she's on the same page as you, financially, spiritually, sexually, etc before you seriously consider letting her encroach on your single man-space permanently.  

For example, my city allows no more than 3 unrelated adults.

Fair housing laws are federal, and then some states add additional protected classes.You can say that someone who is younger than 18 may not sign a lease with you, because they aren't an adult yet.  

@Max Tanenbaum excellent point about the puppy vs adult and the landlord reference. 

I have a 5 bedroom house that I am currently renting to 4 adults and 2 children. 

I suppose for any situation with more than one adult tenant, it would be a good clause to include.

Listing all adult occupants on the lease as jointly and severally liable is the right thing to do.

So, maybe two adults and one kid, vs two adults and at least two kids.And families with extended family living with them, are unlikely to be looking for a two bedroom, one bath home.So, my vote is to leave it alone.