I point you, as I would anyone else, to http://www.sirlin.net/ptw-book/intermediates-guide.html
The "Spirit of fair play" is a misnomer. It's simply a narrowing of a playstyle into something that avoids having to think too much.
It's not limited to Setons players (so no need to get so defensive about Setons), or even supcom, or anything.
Where there's competition, there's players that will say "Yeah, you won, but you did x, y, z, which is cheap so it doesn't count".
While you do that, you don't improve.
As I said, though, it's up to the individual groupings to decide on rules.
If you want to play 3599 with x2 res nr20 no nukes no game enders, then fine.
If you want to play 3609 with everything available then that's fine too. Just tell me when the game starts what it is so that I know on what I am commentating.