We have been using Gildan 50/50 poly cotton and the 65/35 poly cotton polo shirts that we are part of an ongoing order for a running club. We bought the smaller 8"x8" platen for our Adkins beta maxi to do the Logos with. The edges of the foam on the platen are quite sharp so we use a small Teflon pillow. This allows for the buttons and collar to hang of the side and we don't get any sharp edge marks on the Polo.
We are using TMT 123 premium flex with their settings of 120C and 20 seconds with a quick second press with a grip flex pad we bought from Target Transfers. The only thing I might of done different would have been to buy the 6"x6" platen instead.