Keep it as light as possible with the Supersport. I flew mine with an old P-80 that probably put out similar thrust and although it flew ok it didn't really do the airframe justice.

Don't bother with the wing fuel tank and don't add any extras (powerboxes etc) that aren't necessary. The Flash does tend to need nose weight (so put the heavy things at the front) but if you can get it to balance with smaller RX packs then I'd do that. I'd be looking at something like 3000mAh Lipos (or maybe even smaller) rather than 5000mAh Nimh.

Have fun with it. It's a great flying airframe and I may buy another one in the future.