It depends on the school, so I would check each school you're interviewing at what their financial aid application timeline looks like. Most schools have a deadline around mid-March or two weeks after acceptance, so you can wait until after you've been accepted to submit. Some schools have deadlines before you find out about acceptance, in which case you should submit asap.
It is possible to add after submitting CSS, you just have to pay for each school.
FAFSA is free, but you can only list 10 schools at once. So if you have to add other schools, you have to go back and remove schools.