As long as you don't control the exact hours they work, train them, or control anything but project guidelines and due date you can pay what you want, but it's really a better idea not to do hourly but by piece or by entire price for the project. For instance, if a contractor bids too low on a project and they lose money, that's their loss.
However, as a writer, I refuse to work for less than a fair hourly wage for my expertise and hard work. I ensure that I bid appropriately. I do this by estimating the time it will take me to do the project then multiplying that by what I need to earn hourly. In addition, I write specifics into the contract. For instance:
I will write 10 articles of at least 500 words each for $200.00, they will be 100 percent original works, as verified by copyscape.... or I will enter 100 pieces of data consisting of x number of keystrokes for x dollars.
This protects all of us from scope creep and other issues.