October is probably going to be hot and somewhat rainy. Be prepared with rain gear!
There are quite a few cenotes on the tour trail . . . my favorites are Dos Ojos and Grand Cenote, although the halocline effects in Taj Maha are fantastic, and if you get there at the right time, the Points of Light are fabulous. I have not dived Hidden Worlds.
There are lots of good guides. My personal recommendation would be Dennis Weeks (Pez de Diablo here) or his partner, Natalie, because I know and like them personally and find them to be good folks with whom to spend a day, and because I have taken a class from Dennis and know what his approach to diving in the caves is.