Is it quite the norm that I will meet people to tag along with along the way? Others travelling alone?
Definitely. Loads of single solo type travelers on the southern islands and beaches in December. Your main thing will be finding accommodation since it's so busy, although if you're flexible you can sometimes turn up and find a bungalow of some sort. Just with your limited time you might want to look at some bookings.
Lanta is the easiest / least busy of the places in question. Just arriving by ferry and following the touts either on the boat or the pier you'll probably find a place to stay. Klong Dao is the busiest beach but quite couple-family-oriented, Long Beach would suit you well, then quieter spots further down the island. You can ride an elephant down near Klong Jak if you missed it in Chiang Mai.
Ao Nang is a very commercialised tourism town with big resorts and package tourists. It does provide convenience though, having road access to the surrounding attractions (rent a motorbike to explore). Railay is gonna be busy but head to East Railay for the best liklihood of a vacancy. West Railay is nicer but the few places that dominate it are well into the 3000++ range and bookings would be essential in December. Phi Phi is going to be even more crowded so book in advance. Look at the following sites:
http://www.asiarooms.com/thailand/thailand-hotels.html
http://www.agoda.com/asia/thailand.html
http://www.sawadee.com/
