One at a time. Install an add-on, check your game still works, and only then add the next. If several go in together and something breaks, it's very hard to tell which one did it.
Add-ons aren't part of GTS itself: the rest of this site describes GTS as Jay ships it, and an add-on can change any of it. Ask about an add-on in its own Discord channel.
Loading the add-ons…
Started from the GTS Black Book by Hexanode, under the GNU GPL 3.0; this list stays under that licence. Authors, dates and downloads come from Nexus Mods (recently). Curators: ask Tom for your add-on's password, then use Edit an add-on. Not listed? Ask Tom to add it. Making one? See Modding GTS.