Thanks GitarooMan, Annilhilator, and CrazyPanda.
I was mainly wondering because I wasn't sure if I was getting the run around from my local Gamestop. They said I couldn't stack the multiple 360's due to the fact that the 100 trade-in is specific to the first Xbox and is an inflated trade-in value. So for each additional trade-in you will only get the base value.
In regards to trading the VGA cable and composite cable, my specific store told me that they could not do this because they need the cables at the time of trade-in. I told the guy on the phone that I will never use the composite cable, due to the fact that I either play on a monitor or HDMI tv. He said they need the VGA cables for trade-in. I said to take the old 360 as trade-in, let me open the new one, hand me the VGA cable, and I would hand him the composite and we would be all set. He insisted that he needed the VGA cable, and I asked him if he understood the difference. He repeated his original statement of needing the VGA cables. At this point I was getting irritated so I just told him that we could figure it out when they get the next shipment. I wasn't quite sure if he was an idiot, or wanted the VGA cables for himself.
To answer the question of just giving the original cables, I gave them to a friend since his set got destroyed.