Do templates stack?

[Taason the Black]

Meaning say you have the advanced half fiend template(s)

Advance = +4 to all stats, +2 to Natural armor

Half Fiend = +4 to 3 stats, +2 to remaining stats, +1 to Natural armor

Am I correct in saying it would be +8 to 3 stats and +6 to the remaining three? +3 Natural armor?

[KainPen]

yes, but it is not recommend to add more than one template. as the CR is skewed. You have to reevaluate the monster, hp, ac, possible damage and see what the new cr is. It is recommend that you build a monster from scratch if you are going to add more than one. But if you are in a rush, you can.

[Lifat]

Yup. I agree with KainPen completely here. The improved creature will usually not match the CR it'll have.

[Nefreet]

You can pile on the templates if you wish, but as others have said, if this is a monster for your PCS to fight you may just wish to look for something similar that is already printed or make up one yourself. The biggest thing that templates do not advance is HD.

But, for evidence that you can stack templates, you needn't look further than any of the Shaman Druid archetypes. They have the option of applying both the Giant and Advanced templates to their summons, increasing the level of the spell by +1 for each template.

[Bizbag]

Use some common sense when applying multiple templates and make sure the creature can qualify for the second template if they have the first - so probably no vampire liches, because they both require a living base creature; once you have one template you can't get the second.

Generally speaking, though, you can stack them - a vampire werewolf is legitimate in Pathfinder.

[Lifat]

Oh. Bizbag is of course right in this. Generally speaking templates stack, but if the requirements conflict with each other, then they can't both be applied to the creature.
And Nefreet hits the hammer on the nail when mentioning HD not advancing when applying templates. This will put the monster back a good deal of HP and leave it vulnerable on other stuff aswell.

You can absolutely slab on two templates but you need to check up on the resulting creature and compare it to what a creature of the new CR should have. You could either decide to lower the CR or maybe give it other bonuses aswell (unless the creature actually end up being a good match for the CR).