IIRC , the mods are treated as Custom Scenarios. So you would launch the game normally, choose scenario, choose custom scenario, pick the mod you wanted to play and then the region plus options etc.. then play the game. Its all pretty easy.
You can download alot of the mods on Fistalis's web page which is in his sig above.
This is correct, with the exception of my UI modifications. Which will be included in any game you start after installing them. None of those will effect how a default game plays out though. Most of them simply provide functions I thought were needed (easier to see U.N ratings and Manual bond repayments). The satellite menu is the exception but it can simply be ignored if you don't want to use it in the current game since the AI doesn't use them anyway.
All other files are only used by the game when you choose the modern world scenario in custom scenarios or sandbox.
No need to uninstall the mod to play the base game. As a matter of fact I have all my mods installed and can still play the base game just as BG intended.
If your really worried about it you can have multiple copies installed but it takes a minor reg edit to get them both to update when updates are released.(I have 4 versions installed right now.. 1 I use to play 1 thats used as a test bed to make sure I include the proper files in my mod releases, 1 thats running an old version to test issues with my mods people might see using old versions and one clean installation that doesn't get touched incase I really screw something up while modding I can just copy paste these files to the botched version)
Or you can simply back up all the files.. SRCW is rather small in total. something like 900mb for a clean install.(smaller if you zip it up)