I've thought about adding orange into there as well, and running them as the signals, yea, but never went through with it. If you do, then DEF post some pics.
I used 6 green LEDs per side, 3 on top and 3 on the ottom. Although I'm pretty sure a 4" cathode would work really well, it'd just be a matter of being able to fit the transformer in or not. In my Alero I had red cathoeds in like that. I'd worry about the angel eye rings, because they're kind of fragile, but if you can get them in without breaking them I think that'd work great, too! Regardless, if you do it post some pics!