.... There is a section for that, its New Member Introductions & Tips, New Member Introductions & Tips - Airsoft Sniper Forum
While I admire you wanting to use .66 bbs, have you actually looked for them? I have only seen 4 people selling them. Airsoft Atlanta, BBBastard, careful, and someone mentioned someone selling on Airsoft Mechanics I think. Airsoft Atlanta has 40 .66 bbs for sale for $16USD. Why spend $16 for 40 .66s when you can spend $22 and get 2000 .43s?
As for the part about a purpose built gun, not entirely true, but pretty close. You will need a top tier performing rifle to get the most out of the .66s. With airsoft being limited in your area, I advise forgetting about the .66s.
HPA/CO2 is strongly recommended for most cases. It's a flat out requirement for using .66s. No ifs ands or buts about it. It is, period.
I would suggest the KJW over the G&G simply because the KJW can utilize more aftermarket parts over the G&G and all of its proprietary weasel snot.
As a forum rule we (generally) never talk about powering rifles past 550fps, as it is against forum rules and generally deemed "unsafe."