The airport bus will be in operation until to the arrival of the last flight. So, you could catch one to reach Gulin downtown area.
I think impossible for you to hire a taxi to Yangshuo at midnight. If you would like to go there at the same day, I'm afraid that you have to rent a car in advance. But the fare may be much higher! Better stay in Guilin for the night and go to Yangshuo the next day.