My car(G35C) is only lowered 5-10mm in the rear on HKS RS and here is my alignment specs


camber -1.3 LF & -1.6 RF (Factory Range -1.3 - 0.3)
-1.6 both LR RR (Factory Range -1.8 - 0.6)

toe 0.05 both front (0.00-0.08)
0.11 both rear (-0.01 - 0.22)

If I get a rear camber arm only, will I be able to get into nominal settings and keep my toe at 0.11 relying on the factory toe adjustability?
My rear tires don't seem to wear in the outer edge at all, I bought new tires 500 miles ago and the outer edge is totally untouched(no contact).

you should be able to get your rear camber around -1.2/1.3 without screwing up the toe.
-GP-

that's with the aftermarket rear camber arms right?

yes.
-GP-

-1.3 rear camber is nominal I'm at -1.6... that's not far is a 0.3 difference enough to make the outer edge not wear at all?

the outer edge of your tire isnt going to wear at all if your camber is even at -1.0 on both sides. Unless you take corners at high speeds and drift around everywhere. So, no, at -1.3 your outer edges wont wear.
-GP-

So with my alignment settings, a rear camber kit isn't necessary ?

Seems like I need a front A-Arm more than a rear one?

That would be the best thing to do. The rear is within factory specs. The front isnt. Get the front camber adjustable A-arms and get your front camber to spec.
The SPL is a great kit, i've had the pleasure of adjusting it on one of the G coupe's i work on.
-GP-