My plane arrives at 0055 - anything to do/be aware of at the airport?

How long you coming for? Where are you staying, I'm gussing no room already booked?
But Taxi's yes you should be able to find a taxi without issues. Just look out for the legit one's, info posted elsewhere on here.
Hotel's yes although the doors are locked they will open the doors to let you in and rent a room if not already booked(or full).
That time of night there still some clubs open downtown but failing that, plenty of bars n clubs open in the backpacker area. Plenty of cheap hotels too.

My plane leaves at 00:55 tonight for Doha

Crazy Buffalo at De Tham and Bui Vien never closes and a few other bars near it I will bet are also open.

The hotels are used to coming and going at all hours. You should be able to find a room.

There are always taxis at the airport. Just try not to get a fake one.

You don't need to worry about your timing. After all formalities you can come out nearly 1am. If you want just stay there till dawn or pick a Taxi from there and go to Pham Ngu Lao, Bui Vien, or De Tham street where you can stay or pass your time in some bar.
Happy journey!

Thanks for the input everyone. My hostel is booked for the next day in Pham Ngu Lao so figured to just wait til then. I figured as such about the taxis but just needed some additional peace of mind!