umm, those are a lot of questions. Ill try and help with some of them.

your gp extra 40 will be fine for entry level flying. I started with a gp extra .60 kit with a 1.08 so thats not overkill

I would start with properly balancing and trimming the plane before you get to the manuvers. you can find trim charts at the IMAC site or at nsrca.org. I have flown many beginer planes and have noticed a trend towards WAY too much elevator and aileron throws. also that saito might be great on power but make sure you are not too nose heavy.

Basic class does not require you follow scale rules. it is meant as an introductory class to attract interest from guys like yourself. any sport type of airplane can be flown. however, here are the details from the rules: all aircraft must not deviate more than 10% of scale. all aircraft must have a scale pilot and instrument panel. there is a 1% flight score penalty if you dont. but as I said that does not include the basic class only sportsman and up. remember in IMAC we try to emulate the full scale planes and format for competition, hence the rules.

the edge, extra and cap are the most popular planes I think only because that is what the major manufacturers are producing. you are not limited to these planes only. I have flown a staudacher, ultimate bipe, one design in IMAC. right now I fly a 35% extra. at that size (and 40%) the extra seems to be the better plane for IMAC sequence flying, but they all have thier own characteristics and many people have thier own opinions.

I am not familliar with creek hobby products. a 1.60 engine would seem to be a good choice for a 1.20 airframe though. quality servos such as any of the top digitals make the biggest difference in any plane. and you pretty much answered yourself about locating the feul tank on the CG. as the tank runs empty your balance can certainly change if you tank is not on the CG. all my planes have the feul tanks on the CG.

hope that answers SOME of you questions

Hubb