I dunno how the Excellerator is threaded, but that is what determines which barrels it takes, you might find it's threaded for spyder & inferno barrels, which is a common thread for lower end markers, all the big expensive guns have usually got their own specific thread.
As for the type of barrel......as the guys have said in the previous post, 12" or 14" are good general use lengths, as for the make, i'm personally wanting to buy a Dye Boomstick coz i've heard alotta good things about it, very accurate indeed, apparently. Abit dear though.