I know of the settings to change to slow down the rate that things take damage, however, what I am unsure of is how do I have them and make them effective in a mod.
Simply put, do I need to load up anything before changing them, or can i just open the geck, make the changes, and save the esp? Im not looking to make items never degrade, but I would like to slow it down considerably.
You could either directly edit the fDamage* GMST's or use FOSE's Con_SetGameSetting to manipulate their values on-the-fly. The latter works out better if you want the changes to be dynamic or alterable through a menu, but they revert to their default values upon reloading, so they must be reset to their desired values periodically to be maintained.
scn QuestSCPTBegin MenuMode 1007 ; Load Screens If GetGameLoaded Con_SetGameSetting fDamageToArmorPercentage 0.10 EndIfEnd
A plugin made in the GECK won't even need to have Fallout3.ESM as a master if it's only editing GMST's accessible in the GECK without it loaded. I'm fairly certain the engine only reads one version of a game setting even if it is reiterated with a different form ID elsewhere.