I just finished installing a MKR-18 on my boat. They are indeed made by Marinco, seems like a decent product. Since the cable extension was less than 5' from my trolling motor to the battery, it only called for 8G wire so that's what I used. No 6G adapter needed. Here's the link to the MK chart for what gauge cable to use.
http://www.minnkotamotors.com/WorkArea/DownloadAsset.aspx?id=3273
Pete