Fucking internet juss ate my fucking post. I'll try to answe you but im pissed off right now.
-Unfinished Business has no compatibility issues
-ditto miniquests
-Dark Side of the Coast isn't compatible w/GOG's BG1 b/c DSotC isn't compatible w/Tales of the Sword Coast, which automatically installs in GoG's copy. Perhaps there's someone who knows how to seperate TotsSC from the main BG1 game who can help you. Otherwise you'll need a retail copy to install Dark Side of the Coast.
-I never used Widescreen or GUI Mod, but without it my game still takes up the whole screen
To maximize your gameplay, install the BG1 Fixpack (forgot which website; google and you'll find it), and BG1 Tweakpack (from Gibberlings 3, where you got the Miniquests). G3 folk are ace at helping. The Fixpack will help your game run smoother (I've had no issues) and the Tweakpack will give you better game options (ie- you can wear multiple Protection Items & wear them with magical armor; this is in line with true AD&D rules, and they give you other options like that as well).