what is the maximum impedance headphones that can be used with the ipod without damaging the h/p or ipod?

I don't have an iPod, but I would be surprised if there was a maximum impedance headphone. If you take the headphone out of the jack the impedance is effectively infinite.

You may be more concerned about a minimum impedance which in theory could draw excessive current and damage the output stage of the iPod. I would be surprised if this was possible given that most modern audio gear can protect against a short circuited output.

The most relevant concern might be that lower impedance headphones may cause the iPod output stage to distort (clip/current limit) at relatively low volume.