Is there any way to use SnOOpys terrain graphics (or any other mod) as the standard game graphics? So when starting a game via quickplay/new game i dont have to waste time on the scenario loading screen?

If it's just Terrain graphics, replace the original graphics with Snoopy's. (Don't forget to backup the original)
If it includes a scenario file, you have to replace the original civ3.biq (for conquests, .bix or .bic for PTW or Civ3) Again: Make a backup of the original file.