Yes an Emerson Fast Core is a really good choice. Its getting more and more common but for good reason. Its comfortable and versatile. The straps are easily adjustable and the head lock system lets you adjust the helmet to fit perfectly on your head. The ARC rails on the side are a huge plus, since you can mount other things like lights, cameras, headset adaptors, Eye pro adaptors etc. The helmet itself also comes with a bunch of pads and a metal NVG mount. The price is very reasonable for the quality and stuff you get. Here's mine:
As for your question regarding medium plates etc, I use a Ferro concepts FCPC sized for medium plates and it suits it pretty well.
Here's a pic and a vid that I made:
2013 Airsoft NSW/Devgru-ish loadout - YouTube