Well, for servers, of course it is very common to wear pants under the cassock. The rules governing what they wear are considerably more ambiguous and less technical than the rules governing what priests wear. I do think it would be odd for servers to not wear pants or something under their cassocks, unless one owns his own cassock. A priest’s cassock is (in principle) also his every day house dress.
My point isn’t so much about not wearing pants under the cassock as it is about not seeing what is worn under the cassock, unless it is socks. Although, as I have said, if the cassock is worn by a cleric as his everyday street clothes (house dress), it seems weird to wear pants with it anyway. The cassock is not, in principle, outerwear. It is not a coat, a cape, a sweater or a jacket. It is a full-body garment intended to be able to be worn in basically any condition within reason, functioning on its own or with a piece of outwear (greca or tabarro).