"Most small startups (by which I mean fewer than 20 people) don’t have enough work..."
What? WTH kind of startup is short on stuff to do? If you have 2 general-purpose devs, they're going to be 100% busy. if you have 1 front-end and 1 back-end dev, they're going to be 100% busy. If you add another dev, that person is going to be 100% busy, whether they're front-end or back-end.