From novarossi Website

C4S - "Special" hot glow plug
engines from 3.5 to 10 cc

C5S - "Special" medium glow plug
engines from 3.5 to 10 cc

C6S - "Special" cold glow plug
engines from 3.5 to 10 cc 25% nitro

C7S - "Special" cold glow plug
engines from 3.5 to 15 cc over 25% nitro

C8S - "Special" extra cold glow plug
For engines 3.5 to 15cc running on more than 25% nitro

so basically higher number = colder plug which is acceptable for more nitro. I'd try the 6 first since you're running 25% and it's what I run in my engine with 20% See how that works. The tune may need to be adjusted slightly. I had a #6 in there and then swapped to a C6S and it idles much higher and stronger (hotter plug I think).