I don't know how others feel about them, but I often sport my linen "Jacobite" shirts in the warm weather. The linen is cool, the lace-up collar is open, and they look good with the kilt. I have both the puffy-sleeve version and a sleeveless one I use when it gets really hot. I originally got the sleeveless for wearing when I run, since I run races wearing a kilt (good ventilation, no friction, nobody comments on how slow I am). But I've found it works well when I'm just walking around, too.

I have a couple of collarless, very lightweight cotton-linen type long-sleeve shirts I bought while traveling in the sub-tropics (Cambodia and India). They also work well for hot days with the kilt, as they combine cool breathability, an open neck, and sun protection from the sleeves.

I have polo shirts and wear them with my khakis. I haven't done so with my kilt, but don't see anything wrong with it. Ditto with t-shirts. I really try to treat the kilt as regular clothing rather than a costume. But the way I dress normally might strike some as "costume-y" anyway, so take that for what it's worth. I can say that in winter time I have no problem wearing turtlenecks with my kilt for casual attire.

Taking a few cues from the military of a century ago, I'm planning to get a couple of all-cotton safari/tropical uniform style shirts in khaki to pair with a pith helmet for next summer. I think these will look pretty "cool" with the kilt. We'll see...

Andrew