Do we need to run a campaign or can we hire these users without a campaign?
Both still counts as running a campaign, only in one option you're running a private campaign with preselected participants and in the other you will be running a public campaign like the regular ones you'll see in the service thread.
About which users to select, that depends on what expectations you have of your campaign and of the style of posters and members you're looking for. From your list, it looks like you're prioritizing quantity of posts made weekly.