Actually you have two, one male and one female, since the bulk of your examples don't show the models face (only a few) I don't see where it would make much difference, then again if you were going to add photos/examples of your line where the models face was shown yes I'd mix it up.
It might make a difference if you wanted to illustrate that your shirts look nice on a variety of body types. Otherwise I don't think it much matters. It can often be a good thing to have one face or person that is identified with your product.
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