Congress can only exercise the powers that the constitution grants it. In some areas (like the ACA mandate) there is legitimate disagreement about the proper scope for that grant of power (does commerce clause allow Congress to pass laws regulating economic inactivity? I think Raich and Wickard were wrongly decided too, but the fact that I don't disagree with the law doesn't give me license to disobey it).
There is no meaningful dispute that Congress can impeach its own members. It can't. The impeachment power may be exercised over the following persons: (let's read it together now, Article 2 section 4): "The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other High crimes and Misdemeanors."
senators (and ex-senators) are not Article 2 "civil Officers." This clause is commonly understood to apply to executive branch persons appointed by the president. It also applies to federal judges, who are (guess what?) appointed by the president.
Congress members are not subject to the impeachment powers of their own house, because the constitution doesn't grant that power to the House or the Senate.