It really depends of the use you are going to make of the model.
Here it's intended for rendered animation, so i would keep them separated and link armor parts to the bones, or for more precision and realism, i could make them part of one whole mesh, but apply a physics modifier on parts like shoulders, keeping some vertexes attached to the skinned mesh so it just don't go "fly" away, and have cloth physics or rigid body physics running on most of the pauldron, for that i'll need to add another belt around the middle of the biceps so it does't just flaps in every way possible even if it's attached to the chest plate.
i could definately attach / make the back plate and front plate a whole object tho.
If you would use this model for video game purpose then it would be best to fuse everything together and make bones to animate every parts you want to move, and export skin , blending shapes, animations to a program like unity.
RE: Chestplate Tutorial part 2 : Modelling the back part of the armor