Your Active Pokemon is placed on the field before you draw any cards from a mulligan. You're actually supposed to first put down your Prizes, then wait until your opponent has put down an Active Pokemon before you draw extra cards, keeping count of how many times they performed a mulligan. Once your opponent has set up, you then draw the extra cards, if you wish. If any of these are Basic Pokemon, you may place them on your Bench, so long as it is not full, before the first turn of the game. They cannot replace your Active Pokemon. So, your Active Pokemon must come from those first seven cards.