Hey! I’m sure you’ll have a wonderful trip. In case you haven’t gotten all your answers yet, I have some advice…. hope it helps!
On Weather: October in the South will likely be pretty wet… rain for part of almost every day. But it’s not likely to rain all day, and it may not even rain every day—there is almost always some clearing. The humidity will be high, but the temperatures will not be at their hottest, so you’ll survive. And air conditioning is very common, even in beach areas, assuming you’re staying in a hotel rather than a hut on the beach. If you’ve been to Florida in the summer, you’ll have some idea of what to expect in terms of humidity.
On wardrobe and other concerns: Thailand is a very modest, fairly dressy place for the most part. Dress codes on the beaches are a little bit more relaxed, but if you want to make a good impression, you should aim to be more reserved than you would be on an American beach, or in an American beach town. Thai friends regularly tell me how terrible they think westerners look and act when in Thailand.
Despite the heat, Thai people don’t generally go out with bare shoulders (like tank tops), or in shorts, and many Thai women even swim with t-shirts on over their bathing suits. You don’t have to go that far, and you can get away with a lot as a Westerner, but if you want to avoid offending people, cover up more than you usually would. Consider wearing t-shirts with sleeves instead of tank tops, and skirts to the knee or longer instead of shorts. Locals will like you more if you do.
Please don’t go topless. Lots of people do, and nobody screams or tries to stop them, but it really offends people. They’ll think badly of you, and you probably won’t even notice.
Thai people tend to smile and laugh a lot in situations where they’re feeling uncomfortable, so it may look as if people are enjoying themselves when they’re actually shocked and horrified.
You can get away with beachwear in restaurants if they’re right on the beach itself, in the open air, without a roof, but if you’re going into any kind of a building, cover up. Bring a sundress.
As for revealing clothes in clubs, opinions are changing. Young, rich Thai people in Bangkok are starting to dress more like their international counterparts—sexier, basically—when they go out clubbing. But many or even most Thai people are still offended by this. As a westerner, in a place surrounded almost entirely by drunken westerners, you can get away with a great deal. But don’t try to wear your club gear during the day, and please understand that ‘getting away with it’ doesn’t mean that people think it’s OK. They’ll just be too polite to object to your face.
The full moon party is a pretty freewheeling environment, but most Thai people—at least the great majority who aren’t making money off of it, or aren’t hardcore party people themselves—also find it fairly disgusting.
On feet and shoes: Sandals or even plastic flip-flops are fine basically everywhere on the beach, and most other places. In the presence of monks, in the Royal Palace, in museums, and in any other government facilities or offices, closed toes are more appropriate. Shoes are removed in temples and homes, but not in most stores or public spaces. Don’t point your feet at people or things.
Anyhow, that was probably more information than you needed, and I probably sound like an old fuddy-duddy. But you’ll have more fun in Thailand if locals like you, and they’ll like you more if you behave a little more like they usually do. Have a great trip!