I've shot a number of charity events and have never needed anything longer than a 400. You'll be good with the setup you've listed. The actual polo matches are not very long and they move very quickly. Best to have two bodies. I'd recommend the 300 w/ and w/o the 1.4 (depending on where you're standing) and the 70-200. You'll want burst speed, so skip the flash (you may want it to shoot portraits before/after). Somewhere in the center of the field of play is best. A few other tricks, focus high (literally) with an aperture of f/4 or higher during the match. There is a lot of room between the horses/riders and the background, so bokeh is not an issue but DoF is. If you shoot center point and f/2.8, you'll get the horse's head in focus and not the rider. Also, be prepared for the divet stomp, which is an event halfway through when they are changing horses. Everyone goes out onto the field of play and literally puts the divots back in place and stomps on them. You'll want to be in the middle of the action with a wide lens. Enjoy, bring the monopod and stay hydrated! It could take hours for the actual match to begin, but it's worth it.