I'm looking now to hire someone to run SMM and also someone to do content writing and some other marketing, how do I know who is the right candidate? And should I prefer someone who does it all together or rather find individuals who each of them know their "areas" best?
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"