[250.50]. If you said "separated," you're wrong. All grounding electrodes must be bonded together. And actually, it isn't possible to "separate" electrodes. Read IEEE-142 for a comprehensive explanation. Before doing that, review Kirchoff's Law of Parallel Circuits and look at soil resistivity data.
 

[250.52(A)]. There are actually eight: metal underground water pipe, metal frame of the structure, concrete-encased electrode, ground ring, rod and pipe electrode, other Listed electrodes, and other local metal underground systems or structures.
 

[250.52(B)]. Metal underground piping systems. However, these must be bonded to the grounding electrode system to prevent flashovers.
 

[250.52(B)]. Aluminum.
 

[250.53(B)]. Six feet, assuming 10 foot rods. The reason is shell theory, which basically says that if the rods as the rods get closer than a certain distance they cancel each other out. While the NEC states six feet as a minimum, longer is better. So, for example, if you drive a second rod and you have 35 feet until you reach the end of the wall you are working near it's a good design practice to go 35 feet between rods. The extra copper in the ground, if buried properly, will serve as an additional electrode.