You'll get paid the flat rate for any size not matter what the end usage is UNLESS the buyer doesn't take out a subscription but opts to buy it as a one-off at a huge rate, then IIRC you get 20%. Don't hold your breath: generally they'd be better to take out a one-week subscription.
Yes, the same images can be sold at the PP and iStock except exclusive files under 18 months old, which can only be sold at one or the other.
I don't know how it's going to work for iStock. They originally told us it was different markets, then hit their biggest buyers with a newsletter advising suggesting they take out a Thinkstock subscription. That was when I opted out and I no longer care. However, the subscription model is quite restrictive, and the T&C (hahaha) state that images can't be stockpiled but must be used during the period of a sub. (not that there's any way of policing that) Not so good if you have a huge job needing many images one week, but also some flat weeks when you need very few, whereas with an iStock bundle you can buy files as and when you need to.