Yeah, that's the conventional wisdom, that textile is cooler. There's a problem with that: textile can be cooler because of the venting. My jacket has poor venting. My pants have NONE. I have never seen textile pants with zip vents like the jackets have.
So, contradicting conventional wisdom, my leathers are cooler... because they're fully perf'ed. I freeze my @ss off if it's below 65!
As for what to wear under, sorry you got no useful input on the other thread after the thong issue was raised
I have a range of REI polyprolylene undershirts and pants. Long sleeve and short sleeve shirts (a tank too, but I've never used it riding), long underwear and undershorts like thin bike shorts without the chamois. Mine are all pretty light weight. I select based on weather. If it's cooler, I have poly ski liner pants and a poly sweatshirt. Polypropylene is the best! wicks sweat very effectively keeping you cool when it's hot and warm when it's cold.
Some keys to comfort are close fitting undergarments so they don't bunch and cause pressure points, well designed seams (again to eliminate pressure points) and wicking stretch fabric.
REI is a good place to look. There are better brands, like Marmot, but you'll pay much more and REI is good enough.
Sunday it was humid and mid 90's. I wore the perf'ed leathers with a poly tee and the snug poly shorts. Very comfortable. I didn't smell great after 7 1/2 hours, but after a couple of beers I didn't care! (I did shower before retiring with my wife)