1) 4-stroke engines (like the one in question, RCV 90SP) needs HOT glow plugs to keep the filament lit up for all four strokes of the piston. A cooler plug would lose it's glow between ignitions.
Wrong, a four stroke engine only needs a plug that is hot at idle, even then it may not be critical. There are brands of four stroke plugs that are colder than others. A K&B 4C plug is about the coldest four stroke plug I have found.
2) Cooler glow plugs are used in 2-stroke engines for high performance applications, like racing. It wouldn't work to put in a cooler glow plug and just lazily fly around.
Wrong again, any time you increase nitro you may need a colder plug. Especially if the engine has a high compression ratio.
IMO most hobby stores give poor advise except for the most rudimentary. There are plenty of experts here, 90% of them give better advise than you LHS.
That said, why is it so important to be peaked?