You dont need anything else if your mbox has a balanced output. balanced cable is not much good if your outputs and inputs don't support it. Not all XLR cables are balanced however. You have your normal mic cables that are connected straight. Balanced cabling also comes in the 1/4" TRS variety.
Yea that'll be fine, if the inputs on your monitors are balanced there shouldnt be a problem. Have a look for some good quality cables that are a sensible length, having loads of extra cable all just coiled up isn't ideal either (and it'll cost you more!)