In Mighty Morphin', Zordon said it was a rule of being a Power Ranger was that they couldn't use their powers to escalate a fight, even against the monster. Unlike the secret identity rule, there is no exception. Calling the zords first thing is considered escalating a fight.
they couldnt do it with nighloks because they would grow anyway, having a second life
And besides, that would take away all the fun. Make it too easy and quick.