What must be remembered when we are looking for a CB partner for Chiellini is that the player must be an organiser.
Chiellini is an excellent defender, but he doesn't organise.
Mellberg and Legro are players of a similar level of ability, but Legro organises the defence and Mellberg doesn't, which is why we look much better when Legro plays.
That is why, as good as Agger and Gallas are, I'm not sure I would go for either of them.
We need a more experienced player, preferably a sweeper type CB.
However, I struggle to think of anyone who fits this description(Carvalho would be ideal, but isn't available), so it may be necessary to go with a younger player (Criscito, Zapata, Agger) and accept the defence may struggle for a while, but that one of the pair will grow into the role.
I think Agger and Chiellini are too similar in style to work well together. The fact that they are both left footed is neither here nor ther.