There isn’t anything in the ratification materials indicating that Congress could not exclude slavery from territories under the Territories Clause if it wanted to. However, during the Missouri Crisis, former president Jefferson argued that the principle of new-state equality established by the Northwest Ordinance meant that new states must be left power to decide this question for themselves, as the old states had done.
Federalists at the Virginia Ratification Convention expressly told the Convention that in case Virginia ratified the Constitution, it would have a right to secede. Besides the appropriate lectures of relevant Liberty Classroom courses, you can find information on these questions in chapter 1 of my latest book, Thomas Jefferson: Revolutionary–A Radical’s Struggle to Remake America. http://kevingutzman.com/books/books.html