Gravel size and mix

Just another newbie asking another newbie question.  It seems that on the 
net, 1to 3 mm is the suggested gravel size for substrate.   Amano in "Nature 
Aquarium World" suggests using a granule size of 3 to 10 mm.  He also says 
that "substrates that contain several different types of sand will have a 
longer lifetime, and arranging them in layers [larger granules on bottom, 
smaller on top] improves permeability". He uses Fuji sand (of volcanic 
origin) in the lower portion of his substrate to "prevent lumping and 
hardening, and to improve permeability".  So, my understanding (limited) of 
this, is that he uses different sand types to prevent compaction (makes 
sense), but I'm not sure what he means by "improved permeability".  Is there 
really a huge advantage to setting up a substrate in layers of different 
granule size, and mixing different sand types?  If so, how about breaking up 
a lava rock into gravel-sized pieces and mixing this into the substrate to 
serve as a substitute for Fuji sand................or should I be spending 
my time doing more important things? Any thoughts?