A Republican should sit in the White House, following Trump. Changing parties is not acceptable, because the country voted for a GOP Presidency. They didn't vote for President Pelosi. So if we move down the line of succession, it should be a Republican.
It would be President Pence if Trump is impeached - the impeachment of the president only removes the president. And Pelosi is not in the line of succession and will only be in it if Democrats win the house in 2018 - and even then, she wouldn't ascend to the speakership until January 2019. With how things are going, Trump might very well be out of office LONG before January 2019.