Allan,
The G/flex is probably overkill for our use in models. The 650 is a bit thick and I wouldn't thin it as you could lose some of its good qualities. The 655 is useless for us, as it is like putty. I'd just stick with the 105 resin with either 205 or 206 hardener. If you ever need to thicken it, I'd use the 407 microfibers. Don't try that for fillets as it is really tough to sand. For fillets, I use either AeroPoxy Lite or Super Fill. No substitutes.
Alan