Get a copy of the InformINIT database, and either Symbiont or Conflict Catcher. Both of those will reveal a lot more INITs and System Folder files to activate or deactivate than Extensions Manager. Reading up on each one in InformINIT will help you work out which ones you can safely disable.

I used to keep separate sets for different purposes - one for MIDI and music for example, that disabled Appletalk and other needless things, and one for internet use which was kinda the reverse. And another very bare set for debugging and repair.