Well that's certainly an option. I've done the monorail from bally's to MGM many times but not since the big monorail was in place. You can get on at Flamingo according to the site, and go to MGM.

Once you are at MGM you can cross the street via the overpass to NYNY and then a walk to Monte Carlo (here is a map at http://www.vegas.com/map.html)

Taxi also an option. I usually walk it. Grab a bottle of water and go.