Red Terror is Fast Attack. If you have enough, you can mutate a few, but not biomorph them. They move 50% faster than other infantry. If you want to keep them in Synapse range, take something with wings (Tyrants or Warriors); however, it's not essential, since they have Ld 10. Most people arm their Raveners with Scything Talons and Rending Claws. Beware, however, that they pay a price for that speed, so they may lose to slower troops in melee unless you have a points advantage in that assault. If you decide to take Warriors with wings, you should give them heavy weapons as well, making them Heavy Support instead of Fast Attack (Wings and certain weapons change which slot they fall into), and also giving them some good mobile ranged firepower, something Tyranids tend to lack.