...I went to one of the local experts here, and brought him the new Elixirs. But he told me that gauge is fine for the United States - but here in Thailand, he warned, you really need thinner gauge, specifically, 11. Otherwise, he said, you're in danger of warping the guitar from the additional tension on the thicker strings.
I do not believe this guy is just trying to sell me strings. He's owned his own store for years. Quite a good musician....
1. He may be a "good guy" and a stellar musician, but neither make him right.
2. He's "sort of" in the ballpark with the dangers of extended high humidity, but dead wrong about "warping a guitar" due to "thicker strings" ...dead wrong.
3. Extended periods of low or high RH will crack or swell a guitar, regardless of string gauge. That said, one gets tell-tale signs, obvious indicators in sound and playability, prior to things going very bonkers. Not unlike getting runny nose or feeling lethargic
before you realize ya got a full-blown cold. Guitars don't just up and crack, implode, or swell into a jellied mess overnight. And all this on a guitar with any strings, regardless of gauge.
4. I'll be generous and say he's got good intentions. But he's still dead wrong. So would I trust a guy with good intentions who draws bad conclusions? Feel free to answer for yourself; I've got my opinions.

5.
You are
completely correct: you are already fastidious in monitoring the RH and keeping it consistent. Use whatever strings brand and gauge that you prefer.
6. Given the high humidity in your region --and apparently low turnover in string sales in that store-- I'd consider getting strings mail order when you need them to ensure they are "fresh" and corrosion free. FWIW, D'Addario packs their strings in sealed plastic to "lock in" freshness ...felt funny typing that

Also, perhaps keep a spare set in the guitar case so as to keep that set in good shape.
7. Oh, and welcome on your first post!

Edward