Tfix does modify the maps indeed (even Tfix lite, judging the files in the archive), but these modifications are just small fixes and polishes. I think you should either install Tfix (lite) before Gold2Dark - or use the New Dark patch instead. The Gold2Dark readme file is very detailed, but frankly, I find it quite confusing what exactly it does and does not recommend you to do.
Yeah, the fireshadow and Little Big World are both Thief Gold additions, not TFix.
Big fan of this and Double or Quits too. Because of the well and the very similar backyard both missions feature, I've always thought that Double or Quits is a continuation of Vanishing Point - which kinda make them a cool mini-campaign. I'm not necessarily a fan of the texture pack these missions use (makes reading the environment needlessly hard, since everything blurs into each other), I do like how it creates a unique vibe.