G'day Mate,
The heat range of the glow plug is dependant on the nitro content of the fuel, not the engine. I run 10% nitro in my OS 160FX, & use an OS #8 glow plug, you won't need a colder plug until you use at least 20% nitro.
The vapour trail that you may or may not see, is dependant on tuning & atmospheric conditions, you may see the trail when it is cooler, & not see it at all, if it is a bit warmer, it is not smoke, if it does actually smoke, on the ground, then you are running way too lean, & burning the oil in your fuel.
Do you see oil vapour coming from your exhaust, when you run up your engine on the ground, it is not smoke, to check this put you finger in the stream of vapour, & notice how oily your finger is. If it was smoke, it would not be as oily, because it would be burnt & black.