Hi Grizzman, and welcome aboard.
Some of your questions need more information.
15 acres is not much land... until you start mowing it! With a 6' mower, it's an all day job, maybe even daylight until dark. /forums/images/graemlins/frown.gif
What kind of growth is in your pasture now? You need to walk it several times. Pick a cool afternoon and walk until you know it well. If you don't have any rocks, stumps, or brush larger than 2", you will not tear up a rotary cutter (RC). You may consider something bigger. I don't know exactly what your Massey's horsepower is, but you really need something in 7-10' width. I'm sure a big tractor like the 261 has enough power to pull a 10 footer. In the best case, (no brush) even a big wide rear finishing mower (RFM) would work. They are light and easy to pull, and a tractor can pull a foot or 2 wider finishing mower than it can pull in a brush hog. FOr example, any tractor that can comfortably pull a 6' brush hog can probably pull a 8' RFM.
A loose rule of thumb is that you need ~5 tractor HP minimum for each 1' of brush hog width. A 10 footer would need 50 HP minimum, which you probably have. RFM's would need a bit less HP per foot.
Start with your box blade (BB) level, top link tight as it sits on the ground. If it gouges, loosen the top link a thread or 2 and try again. You will pick it up real quickly. One note about BB: A lot of work on gravel tends to dull the blade. A dull blade doesn't work too great for shaving undisturbed dirt. Maybe get a cheap rear blade if you intend to use your BB for a lot of other stuff.
Ideally, your implements need be wider than the tire tread. A mower is not that big of a deal, however.
Good luck!