I don't know about July but i went in September which is supposed to be monsoon season. To be honest it just added to the holiday - it rained probably every other day for about 2 hours solid, the rest of the time it was sunny, but it's still warm and quite refreshing after the heat. And it doesn't just rain it pours and thunders and lightens - it's absolutely beautiful when you're on a tropical island, one of my best memories is sitting in a little hut on the beach in a storm at 2am, watching the outline of the rocks and palm trees against the lightening - ammazing.
Oh, and i wouldn't go to Ko Samui - it's like Blackpool, very seedy. I met people there who though it ws great and really cheap, but if you've been around Thailand you'll find it's comparatively expensive and much too touristy. There's so many more beautiful places to go - Krabi and Ko Pi Pi were definately the best beachy places.