You don't use a cooler glowplug in cooler temperatures. You use a hotter glowplug in cooler temperatures. The hotter glowplug will compensate for the cooler temperature, keeping the ignition timing close to ideal. However, 20°F isn't enough to justify changing to a different glowplug; there are any number of other variables that affect the ignition timing from one combustion cycle to the next, so with our little 2-stroke engines the ignition timing is always approximate anyway.
I run hot glowplugs in two engines. One of them is in my winter nitro truck, which gets driven in freezing temperatures, sometimes even in light snow. The other is in a 1/10-scale monster truck with a .18 engine that's a little too small for driving in grass; I had to increase the shim stack to reduce the compression ratio so I wouldn't burn-out the engine, and then I needed a hot glowplug to ensure proper ignition at low RPMs with the lower compression ratio. All of my other engines use medium glowplugs. As far as I can tell, hot glowplugs are a workaround for engines with poor ignition or running in very cold (refrigerator or colder) temperatures; they will burn-out if subjected to sustained high-RPM running under normal conditions. Likewise, cold glowplugs are only necessary for engines that run at high RPM continuously in hot conditions and need a glow coil that is extra thick and durable to handle the stress. Medium glowplugs are the best option for general-purpose driving. You can adjust ignition timing by swapping glowplugs, but I haven't found much benefit from that; once you've gotten an engine dialed-in, all you should need to do to compensate for ambient temperature is to adjust the HSN fuel mixture a little bit to maintain a consistent operating temperature from one day to the next. Everyone has their own version of engine-tuning voodoo that they swear by, but I haven't needed to replace a glowplug in a couple years, so I must be doing something right.