For enduros, ALL cars should be started based on qualifying times. We all know there are ITA and ITB cars that are faster then some ITS cars, so you can have the same "ready the safety vehicles" by putting the various groups together. Besides, since this is an endurance race, all of the groups will be mixed anyway after about ten laps or so.
So make it easy... grid based on times regardless of class.
Some other thoughts that might help...
1) Specify in your rules/supps that the qualifying driver must start the race. This keeps you from having the "fast" driver qualify the car but the slower driver actually start the race (another potential "saftey car" scenario).
2) Have your steward or starter talk to the front row about the pace lap... make sure the pole sitter understands they should maintain the speed set by the pace car until the green flag is displayed.
3) Specify that racing starts when (and only when) the green flag is displayed.
4) Specify that the race ends when the checker is displayed, regardless of elapsed time on the clock. This eliminates the potential for protests if your supps/schedule say "3 hour enduro" but you checker at 2 hours and 58 minutes.
5) Make sure all drivers understand what you determine a "jumped start" to be. Most series say that ANY peaking (moving out of line) is arbitrarily a jumped start. Some use "if we see a full headlight, that is a jumped start" rule.
6) Make it clear that you will wave off bad start formations and that you will black flag for jumped starts.
Don't ask me about passing under yellows...
Hope this helps some.