In 1818, did the slave states held most of the seats in the senate.

No. It was a tie for majority of 1818. There were 10 slave states (Delaware, Georgia, Louisiana, Kentucky, Mississippi, Maryland, South Carolina, North  Carolina, Virginia and Tennessee), + 10 free states (Connecticut, Indiana, Massachusetts, New Jersey, New Hampshire, New York, Ohio, Pennsylvania, Rhode Island, and Vermont), so each group held 20 Senate seats. 

Illinois was admitted as a free state in 1818, and its first senators took office on December 3, 1818, so that point, the free states had 22 seats and the slave states had 20 seats. It went back to a tie when Alabama was admitted in 1819.

Because broadest terms allows the law flexibility depending on changing society and circumstances.