User blog comment:Omega natsu2/Chapter 360: The White Inheritance, Predictions./@comment-24119374-20131110141920/@comment-7418318-20131110145356

I don't think mimicking non-humans is a problem, but as Miskos pointed out Gemi and Mini can only mimick beings that have equal or less magical power as their summoner.

But since Jackal is almost asuredly stronger than Lucy, that won't work.