Hi there,
A full plate doesn't allow you to add dex, but what about a character with 8 dex and a full plate?
Does he get -1 or it doesn't count since he has no dex bonus?
With full plate, there's a Dex Cap of +0. The rules say Dex Cap is the maximum modifier you can have when wearing that type of armor. -1 is less than +0 therefore you get that modifier. Same as if the cap were +3 and your Dex modifier was +2... you've got less than or equal to the cap, so you apply it.
Rules quote:
Dexterity Modifier Cap (Dex Cap)
This number is the maximum amount of your Dexterity modifier that can apply to your AC while you are wearing a given suit of armor. For example, if you have a Dexterity modifier of +4 and you are wearing a suit of half plate, you apply only a +1 bonus from your Dexterity modifier to your AC while wearing that armor.
You would still take the -1 to AC though on reflex saves to to avoid damaging effects you would replace your -1 from dex with the +3 from bulwark.