There is no reason to add any Templar abilities to Tactics.

That 15 points makes a difference, if you are using it constantly. I am usually setting Assault to target more robust enemies, because it can drain Stamina from other talents.

Use Shield Bash as default talent (closest enemy).

If you have patched your game from Nexus, then you will have to decide if you want the constant -20% damage reduction from Shield Wall, or just set Alistair to use it against Boss level enemies.

If you are playing unpatched game (meaning that you have not added fixes from Nexus), then just use Shield Wall all the time.

Set him to use Shield Defence in melee and Shield Cover against ranged attacks.

Both characters can use Taunt, but Alistair is usually a lot weaker tank than Warden, which causes your team to waste healing spells and pouches, if enemies are swarming over two tank characters. If your own character is a tank, then Alistair should use Threaten instead and your character should use Taunt.

If Alistair is your only tank, then set him to use Taunt often, or use it manually.

Mages and rogues require more work than him.

Dragon age origins advanced tactics