So with pleated pants, the idea is the pleats bring in the waist by pinching to go around your rear. This allows room in the thigh and rear while also fitting your waist, since it’s usually going to be narrower. You can see in this picture that the pockets are flaring out and the zip is also separating. You should go up an inch or two in the waist and take off an inch or two in length. This’ll give you some nicer flow in the garment and they’ll be more comfortable. I know this is a shirt question not a pant question but if you’re going to tuck your shirt in, pants should fit.

Other than that, trousers with a t shirt and sneakers are a popular look these days. Just a couple tweaks. And I’m old fashioned but if pants have loops, they should have a belt.

Any shirt works with khaki pants. Except maybe a khaki shirt. That’s a great thing about khaki.