if you are going to do any eye mask, i would suggest putting your eye serum on after you are done with your mask. your moisturizer and serum should be the last step in your routine. there is no reason to worry about irritation, but if you want to get all the benefits of your serum, you need to put it on last.
if you are only 20, you don't really need a serum for your eyes. but it won't hurt your skin if you want to use one.
oil free does not necessarily mean that it won't clog your pores. you need to use a moisturizer that is "non-comedogenic".