"Evidence" is irrelevant. All that's needed is enough dirt to produce a reason for impeachment proceedings. Impeachment is a political process, not a criminal one, so rules of evidence don't apply.
Impeachment is a LEGAL process targeting The President, Vice President and all civil Officers of the United States who shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors. (U.S. Const., art. II, Sec. 4).
The rules of evidence most certainly should apply. I don't see why they wouldn't. I would hope that judicial precedent backs me up but I'm not sure about that.