Is the pack an Etech? Well anyway a 300 c (D) will probably be fine on your pack on an 11x7 propm but the 12X6 will probably hurt it if you fly too much full throttle. It all depends on the amps you are pulling and the C-rating of your cells. I havnt tried 1200s but have used TP 1320s 2s, Kokam 1500s 2s, and Kokam 1050s 2s times 2 and they all worked fine, I like the 1050s the best because I use two 2s packs with a homemade y-connector and have one pack on each side of the fuselage and it really helps balance the plane