Both the Benoni and Grunfeld transpositions are relatively healthy, I think.  It depends on what you're aiming for: the Benoni transposition is more positionally risky, but perhaps also has more advantages that can pan out for Black.

Er, no it doesn't. Its an indication of some sort of health (perhaps) but not a lot more.

No opinion as to the lines health or not myself, but if those two say non trivial then I'd very much believe them.

Kasparov in his books against Karpov he doesn't have the opinion that in the summetrical fuanchetto Grunfeld Black equalizes easily. Also Shirov in his Grunfeld DVD shares the same opinion.

After a deep research I've come to the conclusion the way I want to play the Fianchetto KID is trying to transpose to a Modern Benoni (more dynamic play and better figures than the Panno or any variation of the Classical Nbd7) following Dembo's / DW reccomendation with a quick c5. As I think that the fianchetto-KID should be studied together with the English-KID,  if white plays 1.Nf3 & d3 what kind of Symmetrical English (I used to follow Dembo or Bologan's recommedations vs. King's Indian English but they don't fit with c5) do you recommend me (and what books to consult)? One with a quick d5 in Grünfeld style, may be? Thanks for your help.