Consider this:
When you give out XP, the CRB states that you give the same XP to the characters whether you have 4 or 5 characters. The addition of a 5th character doesn't change the balance enough to give less XP per character. Link
With that, though, adding the advanced template might be too much, not only too powerful for the players, but also too much work for you. Just up their HP a bit. Most monsters have average HP, which is right around 50% of their potential max. Just make it 60-70% of their max. That should require a few extra attacks to kill the monsters, which takes into account the addition action economy the players get with an extra character.
Also, remember that ROTRL is a fairly deadly and dangerous adventure path. It might not matter that you have 6 or even 7 players.
You could do that if you wanted to, but be wary. If you notice it's too much, then scale it back a bit.
Just remember that because players get the same amount of XP whether they are 4 or 5 characters, then a 5th character doesn't alter the balance that much. You could easily run the AP without altering anything, and they'll still have a very challenging game.
I have 5 players in my group, and we've TPK'd on both Jade Regent and Carrion Crown. The GM didn't alter anything at all. ROTRL is supposed to be deadlier than both those APs.
Have you considered just throwing out XP?
Event based leveling works rather well in APs.
This is very true, so if you don't want to deal with XP, this would be a good path to take.
I was only using XP to show that a 5th character doesn't change the XP chart, and as such, shouldn't change the difficulty level for the players.