When it comes to hiring folks for SMM, keep in mind that while you can teach industry knowledge, you can't teach personality and common sense. Try taking a look at their public profiles -- how are they interacting with their own following? Is this a person you want to be the public voice of your brand?

In regards to the question about specialization, it's hard to say without specific knowledge of the segments that you are looking to target. If it is too much for one person to wrap their head around, it may be best to share responsibilities between a number of people as you mentioned. My only concern would be consistency of your brand voice.

Hope this helps!