Impeachment cannot be for acts committed before office holder is inaugurated.
Actually impeachment can be for anything congress says it is, the term "high crimes and misdemeanors" is not defined anywhere in the constitution or criminal/civil law. Congress could declare the simple fact a president is under investigation a "high crime or misdemeanor", heck in theory congress could declare the president failing to smile enough a "high crime or misdemeanor". It is doubtful the courts would get involved for separation of powers reasons, because impeachment is considered a power under the sole authority of congress (IE the courts can't impeach the president only congress can).
Practically speaking, I believe impeachment can be for whatever Congress decides it’s for.
Impeachment can be for salacious character.