1.) Not sure I'd use velcro unless super strong, but on the other hand, I think you have to be careful of mounting the amps to the chassis of the car because that can cause grounding problems.
2.) No, don't run separate power wire from each amp to the battery. Run 1 wire, then connect that wire to a distribution block mounted somewhere near your amps, and run two wires from the Distribution block to your amps. As for the ground wires, I also run one ground wire from the chassis to a distribution block and 2 wires to my amps, but I might be changing that to runnning two separate ground wires to the chassis, I'm honestly not sure if it matters.
3.) Already answered above.
4.) Well, I have never made a big deal about this in my car, but I believe the proper way to determine the size fuse you should use is the total sum of the amps of the fuses in your amplifiers. That is the size fuse you should use inline in your power wire. In other words, put that size fuse in an inline fuse holder in your power wire no more than 18 inches from your battery.
5.) Ok, people on this board may say to use 1/0 guage wire, because it seems like that is the size wire of choice. Honestly, I am running 4 guage power wire to my d-block and 4 guage to my sub amp which is twice as powerful as yours (Crossfire vr1000D (1000w x 1 at 1ohm) and I am running 8 guage to my crossfire 2 channel amp putting out 150 x 2 and I have never had any problems in four years and my subs are loud and sound great. So, this was a long-winded way of saying that regardless of what some people might say, 4 guage wire is perfectly fine for your setup.
Oh, and finally, sounds like a nice setup. good luck
