[SixOfSpades]

Installing ToSC in the middle of an existing game will not cause any problems--except with the map. The "old" map won't know what to do with the "new" map areas added with ToSC, and so you won't be able to travel there. To fix this, run the program called MCONVERT that you'll find in your Baldur's Gate directory after installing ToSC. That will update the map, and everything will be hunky-dory.

As far as people in your party getting into fights, certain people just plain don't like certain others. As a general rule, it's not a good idea to mix people of Good and Evil alignment in the same party. This actually makes a great deal of sense: I can't picture Khalid ("I don't mean to be confrontational, b-b-but could you be a little less.....evil?") happily marching in the same party with Xzar ("I wanted Infravision like the Elves....but 'tis more than just taking their eyes.").