So there was a bunch of talk about how you can replace the rear springs with front ones off of an older accord. What are the differences of front springs on various subarus? Any of them taller than others?

I dont have specific specs on hand. they are floating around the web somewhere. I recall that the wagon and RX springs are the tallest with the highest spring rate.

I would not try to go any taller than that to prevent you camber and C/V angles from getting to wacky.

subarustyles put coil overs from a gen 4 Jetta/golf on his to provide a drop. they could be adjusted up as well to some extent.

Is the a difference in length between 2 and 4wd models?

Yes, 2WD springs are lower than 4WD for obvious reasons.