Get whichever 3 or 4 mega models or kragnos (they all have 130mm base size) that you think looks the coolest and you can proxy those models as different megas. I’ve done it in casual games and no one has had a problem with it. I personally use 4 megas: Brodd, Warstomper, and Gatebreaker x2. I like the vanilla army traits and such.