Hi Jerry,
Addressing and the order in which you plug your fixtures in doesn't matter at all. It is like a computer network, the order in which the computers connect to that network doesn't matter. They are still all connected and separate from one another.
Likewise, with your next question, if you simply remove one or more of those computers, it will not effect the computers still on the network. You can add and subtract as much as you like and still run programs just fine.
For example, a friend of mine has 4 PS250s and an SD3 controller. He sometimes likes to rent some more fixtures from me for larger shows. He has programed my fixtures into all the scenes and shows on the SD3 that run the PS250s. So when he uses my lights, he simply addresses them and plugs them in and it is like they were always there. Then when he doesn't have them, it has no effect on the PS250s at all.
The one thing you have to be careful with is if you add a bunch of fixtures to your programs and then try and add different types using the same addresses, you will have to reprogram. But simply adding and subtracting from your standard setup shouldn't be an issue at all.