1) For campaigns that are packaged properly, go to the MOD tab of the launcher and point it to the directory the campaign is located in, then select the campaign from the campaign room in-game.
2) If you used the online installer to get the game, you've already got several of the campaigns, but far from all. If you just downloaded the executable and did a manual install, you did not get any extra campaigns.
3) You can install as many as you like, and switch between them using the launcher. Using a different pilot for each is recommended, though, as you will otherwise have to abandon the current campaign to start a new one.
4) There's a campaign list located
Here, in the Missions and Campaigns forum.
5) No need to load FS1. The
FreeSpace Conversion contains everything needed to play FS1 campaigns, as long as those have been modified to work on the FSO engine - You can find some of those in their forum, notably Cardinal Spear and Destiny of Peace, probably two the best fan-made campaigns for FS1.
Hope that answers all your questions
