I don't know about arrancar being able to use shikai or bankai... It seems that the norm is for a shinigami to spend years and years of practice to reach bankai(except Ichigo, but he seems to be an exception with vaizards too, or why try so hard to recruit him). Okay, so having JUST acquired weapons that appear to be zampaktos, how would we explain such a quick mastery of something like that, and on a large scale? My guess is that their heavier attacks, ones that Ulqiorra might use, are similar to that mouth energy blast that Yami did, only waaaay more destructive.