*The logical choice for allies would, of course, be other Death units. You can only ally Deadwalkers, Deathlords and Deathmages.
*Corpse Carts and Mortis Engines provide solid allies that boost your casting, which is crazy good for Flesh-Eaters who rely a LOT on magic. Mortis Engines are probably best, since they are tough, boost casting, can deal lots of mortal wounds and can heal your guys. Corpse Carts are a little squishy and don't do much other than buff Deadwalkers units, which you have little use for.
*Don't use a Necromancer. Just don't. None of his keywords work with yours. The Mortis Engine, however, is just plain great.