I would recommend you check what your engine spec's say to use short / long plug then from there it will depend on your weather in your area on what plug you use cold / med / hot
Glow plug choice has nothing to do with ambient temps. The heat range on a glow plug will dictate the ignition timing for the engine which mainly depends on the nitro % and what type of fuel your running. Chances are a hot plug will work well with the smaller engines.
ok thanks, i'm runing 16% and have just bought no.4 hot plugs?? Does anyone know the spec of the GT-12 I.e power and top RPM. iv been looking on google for ages but haven't managed to find anything.