well in response to why paul thinks everyone is changing sides. There have been a few players claiming they will being turning renegade in the next mega battle, so I can see where he is coming from. I am also of the same mind set, if the order side seems down a few guys I will bulk up the Tau and see how they hold up for the greater good.
I stilll honestly believe that if we limit the heavies to the amount of players we might find the situation balances itself. I mean for example order, if there are 12 players and 5 of them don't have any super heavies they wanna bring well then there you go Rob can bring his Titan legion.
As for fluff as long as the side as a whole makes sense I can see it. Like a legion of titans sent to back a marine force is believable, ie Rob's force is there to assist the normal composition of space marine players that will show. Like the dark angels or ultra marine.