They are in between. A thinner material than the usual athletic socks, they are ultra soft and smooth so they don't add bulk to shoes which are already well-fitted. I love that they have a silicone insert on the inside at the heel of the sock so that they don't crawl down into the shoe when you walk.
I wear a women's 8.5 - 9.5 depending on the shoe. Most flats and sneakers are 9 or 9.5. I have a wide foot. These are snug on my foot but not uncomfortably so. Any more of a squeeze might be though. If you like your socks tight go for it..
They should be fine. I wear all types of slip on Sketchers and they never show...especially if the sock color is similar to the shoe color or your skin color. I LOVE these socks! They never show and they never slip off your ankle...just don't put them in the dryer so the elastic doesn't get ruined by the heat.