Q1: I assume you understand that this is something only you can determine, based on the devices you would like to use. You'll have to look at each device's specifications for address assignment capability.
Q2: 6" shouldn't be a problem but 4' could be. I2C is really only intended to be a short distance internal bus. Mostly on a single board or within a small case. There are other communication specifications and protocols for reliable communication over longer distances, for example RS-485 for the hardware bus.
Q3: It's possible to put terminators just on the master device but you might have to change their values as you add or remove slave devices. I2C terminators are actually mainly pullups, since the drivers are all open collector/open drain. They aren't so much to match impedances to minimise reflections.
Here's some fairly technical info:
http://www.edn.com/design/analog/437129 ... unications