safe to take a taxi from aiport at nigh in Mexico city?

Just make sure you buy a taxi ticket inside the airport. Then let one of the official airport taxis take you into town. Don't take an unmetered/unlicensed/unmarked taxi. You'll be fine, and as you've prepaid your cabfare, the driver won't be able to cheat you, fare-wise!

If you are going to one of the real budget places that don't have doormen or keep the door open late, you might want to ask the driver to wait until someone opens up.

The driver may try to convince you that he knows a hotel that is better than the one you want to go to. One time, when the driver was very persistent and I was travelling alone, I eventually told him that I really had to go to my hotel because my husband was waiting for me there. That stopped him. He didn't know of course that I have not had a husband for the last thirty years or so.

In Mexico, the fare is all you pay. Tipping the taxidriver is not expected.

thanks for everyone's reply! I really appreciate it! We have arrived at Mexico city with no problem, in fact, our hostel owner picked us up from the airport! how cool is that :)