In April 2011 I entered a lease agreement with my landlord. It was a full year lease, ending April 2012. At the time the people on the lease were:
Myself
My pregnant fiancee
My cousin
My cousin's girlfriend

In April 2012 we decided to continue living here, and told our landlord this. He agreed. We did not renew our lease, at this time the people living here changed to:
Myself
My fiancee
My child
My friend
My friend's wife
My friend's child

Also at this time, because I had paid a deposit previously my friends asked me if they could "pay me back" for their portion of the deposit. I said they could ($300 owed by them).

We continued to pay monthly rent, even without a lease. For personal reasons my fiancee told me that she and I had to move at as soon as we could. So, we contacted our landlord on June 15th, telling him we'd like to move out August 1st. We told our housemates this at the same time.

On June 23rd, we come back to our apartment after being out all day and see that our housemates are moving all of their stuff. I talk to them, and they say that not only are they not going to pay for July's rent, but they aren't going to pay me back the $300 for their portion of the security deposit as well. They said that if I got the money back, I could keep it.

My fiancee and I are still responsible for July's rent, but cannot afford it without them.

Do I have a case against them? For July's rent ($550)? or their portion of the security deposit ($300)?

If I do have a case, which court do I go to? Civil? Small claims?