Evi, I'm not too sure on the Unicorn x Lightning/Water combo to be honest. I've tried using unicorn for many, many limited animals and I've yet to get a limited animal (or a gem creature DURING limited breeding times) through a unicorn pairing. Maybe it's just my horrid luck, or paranoia talking, but I don't think I've ever seen anyone actually get a limited using unicorn in the breeding pool.
Or am I wrong? :\