Ray,
I'll try to answer all of your questions.
First, the difference between "short" and "long plugs" is the length of the threaded part of the plug. Typically engines .19 or larger use long reach plugs while engines of .15 size or smaller use short reach plugs. There have been a few exceptions. When R/C first started most R/C plugs used to have an "idle bar" or small bar across the combustion end of the plug to provide better idle operation. Current R/C plugs may or may not have this bar.
Second, a long reach plug installed in an engine designed for a short reach plug (i.e. .15 size or smaller) might possibly hit the piston at top-dead-center or even prevent the engine from being turned over. If it doesn't hit when flipping the engine over by hand then it probably won't hit when running unless there is a lot of slop in the engine's internal parts (i.e. connecting rod ends, piston pin holes, etc.).
Third, R/C plugs typically are in the hotter range of plugs to, as you've been told, assist in keeping the fire lit when the engine is idled down. However, except for speed or racing applications, using a hot range plug doesn't cause lower peak rpm or result in any meaningful loss in top end power for control line sport or stunt flying. Actually when running your FOX engines it is very beneficial to use a hot plug, even though they may not be R/C engines. Most old FOX engines use an iron/steel piston/liner and, for maximum longevity, need fuel with a larger percentage of oil than required by the more modern design ABC (aluminum/brass/chrome) engines. Compared to a cold plug, the hot plug is more tolerant when running with a high oil content fuel and prevents the engine from quitting when running with a richer needle setting.
Fourth, your FOX .35 and .19 engines use long reach plugs. The FOX .15 uses a short reach plug. I have no experience with these specific engines but I ASSUME the Norvel .15 and .074 would use a short reach plug.
Fifth, I would assume that your literature that specifies a "standard" plug is referring to a plug without an idle bar .
Hope this helps.
DennisV