Let's turn to GQ for some advice.  According to those menswear gurus, trying to rock a polo and an undershirt is a bad idea.  Polos are meant to be worn by themselves as casual attire.  Wearing another layer under them will make you look bigger, and not in a way that's flattering.  Plus, if your polo sleeves are not long enough, you might be able to catch a glimpse of your tee sleeves sticking out.