I think you can't overexpand since your tongue will not grow wider. And tongue is the main factor in expanding the upper jaw.

someone said on another forum too much expansion is bad and there is an ideal . 

Well too much of anything is of course undesirable. But is accidental over-expansion a reasonable fear? I doubt it. It's probably not even particularly possible. Ideal size = ideal function. It should be pretty clear when you have reached good function. You will feel balanced and stable. Everything about your anatomy and movements will simply make sense.

My orthotropist said that if you expand too much on the top the back teeth can start to drop down. It's why she halted my horizontal expansion and is swapping in a new appliance for forward expansion.

What is meant by "Drop down"? Like... out of the gums?

Maxillary molars (and the posterior palate itself) dropping down & forward in relation to the rest of the face is an overall good thing for aesthetics, breathing and anatomy:

- smile becomes wider and more horizontal

- airway gets larger

- the resting position of mandible moves forward and gonial angle increases as the mandible remodels to the new lowered bite -> increased mandibular aesthetics + increased anatomical support

All this is usually achieved automatically with expansion. Placing a pen between molars and biting on it can easily demonstrate how much additional anatomical/structural support increased teeth height gives. So I don't know what your orthotropist was thinking. Perhaps there will come a point where the bite has dropped TOO low. 

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