Its neither legal or illegal... its just cool and different. Do it well and noone will bat an eyelid, but make it obvious thats its just to save on money/painting and you'll annoy a lot of people.
My local GW is challenging all WFB players to paint 250pts of a new army a week between the rulebook release and games day- most of the VC players are using unit fillers in zombie units just to keep up... but it also looks cool. They have zombies, zombie-skaven, zombie-empire, zombie-ogres and even a zombie-dragon (as in a dragon in a zombie unit, rather then an actual dragon) all in big blocks.Its an awesome looking unit because it is just corpses or multiple battlefields just being thrown together into a hoard of undead, as it should be (though the dragon's base had to be altered to be a mutliple of 20mm... he made it 100mm*60mm so by itself its the first 3 ranks of a zombie unit- with enough models behind to replace the dragon if the unit goes under 15 wounds left).