Even without knowing all the details, I can point to several things your landlord did that are improper to the point of being illegal.
"Can he get rent from me after he had someone else move in?"
No. It sounds like you have a way to contact the new tenants. Find out what they paid for the month you moved out. You can legally subtract that from what you owe the landlord for that month.
"Why can't get take the 7 days I did live in the house from my security deposit?"
He can. It's bad practice to use the security deposit to cover the last month of rent, but he is allowed to do that if you owe money when you move out. You should demand an itemized breakdown of how he applied your security deposit. In many states, the landlord is legally required to supply that within 30 days of the end of the lease. If he refuses to supply a detailed breakdown for anything he takes from your security deposit (including owed rent), I suggest you take him to small claims court. You might get back more than your security deposit.