Morphing a Human should be fine, as long as you try not to do something that could get the real person in trouble. Like steal something, or beat someone up, or destroy something.
Cassie thought morphing and controlling Humans was like what the Yeerks do. she overlooked the fact that a morph had no self awareness. ax's body never considered its own existance, or why it was being controlled by something else. It never attempted to speak to Ax. That's a large, and key, difference.