I've recently installed my new Steam version of Oblivion Game of the Year edition and I've gone to making some mods for myself, but there's something I need to know. Since this version automatically installs Shivering Isles, is it still possible to create a mod that doesn't require Shivering Isles to run?

To clarify, if I don't use any content specific to the Shivering Isles expansion in my mod, will people using only vanilla Oblivion be able to play it or will they still need to have SI installed because I created the mod while I had it installed myself?

From what I understand, as long as you don't use any objects/NPCs/Characters/Scripts/whahaveyou that was introduced in SI, it should be fine.

Correct..
IIRC, SI stuff starts with SE in the CS. But be careful about meshes and textures too.

Yes, everything starting with SE and XP (like XPBlood) should be avoided.