Yeah, the Ross foam board is heavier. I built my first plane out of the same stuff. Using heavier foam board makes a plane more tail heavy because more of the airframe is behind the COG than in front. However, your battery is heavier than recommended for that plane, so you can probably still balance it. If you have your battery all the way forward on the bottom of the power pack, and you balance the plane on your fingertips at the COG marks on the wings, is it still tail heavy? If it is, you can move try moving the receiver and ESC forward.
PS, having the heavier foam and heavier battery will make the plane more difficult to fly, but it‘s probably close enough. Weight your plane. The all up weight (with battery installed) is supposed to be about 165g. 200g would probably fly fine. It can probably be up to 50% overweight and still be “flyable”, but the heavier it gets, the more difficult it will be because you will need to fly faster, take off faster, and land faster, to avoid stalls.
Good luck, post videos! 🙂