If your 8700 is using BIS / Desktop Redirector, I'm not sure.
If it's using BES, then when you plug another BB into Desktop Manager you should get a prompt that says "Your messaging account is currently configured to use device with PIN <PIN of your 8700>. Would you like to switch to the connected device with PIN <PIN of your 8100> instead?". If you say NO to this question you can still load applications on the 8100 (and copy music) without affecting the 8700. I wouldn't touch the PIM and email settings though. If you are not using BES with the 8700, I'd wait until someone else says it's OK to go ahead.