They are typically not compatible, but there are hop chambers that allow you to mount AEG barrels.
However, the VSR/GBB cut is better.
As far as length, I was able to produce the same results with a 430mm barrel as a 650mm barrel. And my PTW is able to produce staggering accuracy with a 260mm barrel.
Once the BB has stabilized itself in the barrel, no extra amount of barrel will ever help it become any more stabilized or fly any straighter.
It's up to you, whether you go with 303 or 430, I'm confident the 303 would be accurate, but I can tell you with all certainty that the 430 IS accurate.
As far as BBs, obviously you want the best. If you're running around 400-430, use .30g. If you're planning to run 470-500, then you absolutely need .36g
Between 380fps and 470fps, we found little difference in range with a .30, but the higher fps rifle was actually slightly LESS accurate using all the same parts except for a longer barrel.