i am VERY against the popped collar look. same with double polos. nononono NO.
long sleeves under a polo can look good, depending on the person. i think the best look for polos is just a plain old polo. nothing fancy, nothing underneath. second best is to wear a tshirt underneath. grey polo with white t, blue polo with grey t, etc, would all work. i'd color coordinate them just like with anything else.