5. Nightshades DO like the same soil. It is important to lightly fertilize them throughout the season.
Go light on the nitrogen or you'll get hedges with small fruits. You can get organic "tomato" food at any garden center and it works well for eggplant and peppers as well. I go heavy on potash with the potatoes. I grow most of mine in-ground and we're always flooded with tomatoes. Even after the stalks die off, the damn things keep producing. The most important thing with container growing them is to avoid keeping the feet wet. That leads to blossom-end rot and overall low productivity. In a well-dug raised bed that is rarely a problem.
I can't say I've had a problem with reusing container soil. You still have to mix in some light fertilizer and top dress periodically throughout the growing season.