You cannot play as pro-templar straight away from Act 1, however you can do so from Act 2 onward if you want.
In Act 1 you are still a mage sympathiser, because you have been protecting your sister from templars all your life. You can slowly change that view by making Hawke change her opinion as things develop on Kirkwall (it is easy, there is plenty of blood magic and daemons). As Hawke realises that magic is dangerous and must be controled, then she starts supporting the templars and questioning Bethany if it would not have been better that she had been in the circle.
For Merrill, you can tell her straight away "I want nothing to do with a bloodmage" after her recruitment, just ignore her quests. For Anders, after his act 2 quest, when he shows that his spirit is getting out of control, then tell to gtfo of Kirkwall because he is a dangerous abomination, and he will leave.