Hi Rich,
We only have the September ferry timetable for the moment, and the last ferry from Mastichari to Kalymnos is at 22.30. I don’t know whether the ferry schedule will change in October or not. The timetables are usually announced 2-4 weeks ahead of time (for example, we just received the timetables for Sept. 15-30th today). If you check our website first week of Oct. we will hopefully have the October timetables posted.
If it turns out you miss the last ferry, you can spend the night at one of the many hotels at Mastichari (they are not expensive, esp. at that time of year), on the beach at Mastichari, or maybe at the airport (I’m not sure whether or not it’s allowed).
You can also read the comments after this post: http://climbkalymnos.com/?p=3896
Five or six comments down, this very issue is discussed.
Katie