How do I hire the right person for my SMM? And should I rather hire someone who "know-it-all" or rather a few people who each knows their field well

I'm going to be making some assumptions with the hope that my answer will be helpful to you (if the assumptions are correct) and others.

1. Make sure YOU are "running" all marketing. By this I mean that those you hire should be following your strategy - not creating it. They should be communicating the message you crafted - promoting the offer(s) you created - managing the brand you cultivated.
In other words - don't abdicate marketing strategy to anyone you hire.
2. Depending on where you are in the start-up continuum - pick one in-bound and one out-bound marketing venue. Those will ideally be the lowest investment / highest potential ROI venues based on your market (this presumes you know your CAC and margins). If you are choosing content marketing as one of those venues then run your campaign(s) and keep track of the metrics.
3. Hiring, in all cases, starts best when you know what that position has to do for and in the business to help create value and to increase profits. I like to create a Position Contract that outlines what's required of the position so everyone involved knows what's expected.

With those things in mind - my answer to your question is to hire experts in the specific areas you've identified as the venues you are going to invest in. Get specific and avoid the generalist - at least until you've identified one successful (read "profitable") in-bound and out-bound venue.

Best of luck!

If you can find a social superstar - knows inbound marketing, facebook, twitter, etc. - then I'd go with that person.
A growth hacker would typically know most of these, so they'd be your best best.
http://www.startuply.com/ can be a good place to start looking for people and twitter if you search for things like "growth hacking" and "inbound marketing"

Good luck!

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!