I've never sparred MMA with a new guy, but I've rolled with plenty of big, new aggressive guys. Fortunately, on the ground, they're f'ed, even if we're throwing punches. I'd be much more worried standing up with one of them.
I've only been training for a year, but it's enough to make crazy mofos settle down or quit. Monday, I rolled with a new guy who had obviously wrestled in high school. He was spazzing hard and getting egotistical. I took his back repeatedly, but he wouldn't learn. I tried to tell him how to avoid having me take his back from guard, but his pride was getting in the way. Fortunately, most people settle down and figure out that they don't know jack.