can I cite personal communication from a co-author? I received a strong opinion statement in an email from an expert in the field who later ended up making enough suggestions that I'm including him as a co-author. Coming from me, or from the paper's general authorial voice, the statement would stick out as an unsubstantiated claim, and his papers don't tend to include opinion statements. This would also be the only place in the paper where I'm quoting an expert. Should I just give it up?

To clarify, I'm not even wanting to include it to bolster the paper's argument, in fact it works against the paper's argument. But the fact that an expert has this opinion is a significant piece of information that readers might want to have.
You should not provide information from one co-author as e.g. personal communication since any author on the manuscript is expected to contribute to the manuscript. If there is need to state that a specific author has come up with a particular point you could do that by stating that e.g. "one of us (initials) discovered that..." or something equivalent.

In the end, just remmeber that any fact presented in a manuscript is either due to the authors or to other (referenced) sources. It is rare, but not unheard of, that a co-author may have specific contributions that require, or perhaps deserve, special mention. I would deem this quite rare. Becoming an author on a manuscript means you contribute intellectually to the end product and that includes sharing your ideas.

So to sum up, tred carefully and objectively.

By including a reviewer as co-author, you should follow regular authorship "rules". I fully realise that the question whether or not you should include the person as an author is a different question entirely so I will only focus on the situation at hand.