Can a senator be sued for slander because of things he or she said in a speech on the floor of the Senate?

No, members of Congress are immune from suits against them for anything they say on the floor of Congress. Article I, Section 6 Clause 1 states: ". . . and for any Speech or Debate in either House, they shall not be questioned in any other Place." This grants them immunity from suits for anything they say in Congress.