RX100 versus Canon S100

What do you mean by setting Max ISO?

I am able to set an automatic ISO range (both low and high limits) on mine.
Press Fn button, on ISO menu put it to AUTO, then push 'down' on the rear control wheel.
Then you're free to set both lower and upper ISO limits.

I think this is only configurable as an overall setting, and not individually per mode i.e. P, A, S, M.