Swift makes universal 2.5" ID springs because they're just that, they're universal to span different applications. It's just a spring so multiple cars can use them.
I went with SPC toe bolts to adjust toe because I didn't want to buy new rear springs for my Bilstein coilovers. So I have all of the SPL parts except for the mid link.
As far as spring length for the rear, check out this thread:
SPL Suspension help, KW coilovers V3
ENT-Z used a 2.5" ID 7" length Swift universal spring with his SPL mid links.
Don't even know if I answered any of your questions, but personally I would just get the toe bolts and call it a day. Not sure if Fortune Auto coilovers are "true coilovers" in the rear, but if they're OEM-style, then I would just go toe bolts and skip the mid links all together. That's what I did at least. I figure if you got the super high spring rate in the rear expecting to switch out to Fortune Autos later, it'll probably handle like crap until then because your spring rates are going to be all weird.