You can have the c02 inside or outside- whichever is more convenient for you. Mine is inside, but it's not particularly convenient when I want to change the pressure! But it did save me from drilling holes in the fridge, that's why I did that.
In my full-size fridge kegerator, I can fit 5 kegs plus the 10# co2 tank. I have three taps through the door. I have a WYE fitting on the regulaor. One goes to the distributor that feeds 4 kegs, and one is alone for an extra keg. All four kegs on the distributor are on the same psi from that regulator.