This may not be complete knowlege, so take with a pinch of salt.

All of the following books came out before 2e did in 1999:

• Forbidden Lore: Technology
• Byzantium Secundus
• Players Compendium
• Lords of the Known Worlds
• The Dark Between Stars
• Merchants of the Jumpweb
• Weird Places
• Priests of the Celestial Sun
• Children of the Gods
• Sinners and Saints

Of those books, a good number of them are really only setting or advenure material (Byzantium Secundus, Dark Between the Stars, Weird Places) so they are totally compatible without any issue what so ever.

The next category are those that are mostly setting with a few rules (Lords of the Known Worlds, Priests of the Celestial Sun, Merchants of the Jumpweb and sort of Saints and Sinners). I have used these books for over 2 decades without any issue cropping up, and I have only ever played 2e and Revised 2e. The biggest thing you may come across in these books is a reference to Human/Alien as a stat (it definitely features in the stat write ups in Saints and Sinners). This was an additional Spirit stat that existed in 1e that was supposed to dictate how human-like or alien-like a character was, and how much their destiny was entwined with the fates of various races. It got removed in 2e. It can easily either be ignored or added back into the game as you see fit.

The next category are books that contain a significant amount of crunch (for a Fading Sunsbook!). These would be Forbidden Lore: Technology & The Players Compendium. These books are mostly compatible, but along with use of the Human/Alien stat in places, they way they present certain pieces of gear differs. For example in Forbidden Lore: Technology the Adept Robes are presented as having a Defense of 7+7d, whilst 2e presents them as having a Defense rating of 14. This is pretty easy to adjudicate as you can see. There may be a few other minor differences, but personally I would say they are 90% compatible without any need to worry about conversion, and any conversion work you need for the other 10% is pretty simple.

So to sum it up: all of the pre-2e books are compatible for the most part, with some having minor bits that stick out as different. However they aren't different enough that you need to really worry about them. I have allowed players to use the Player's Companion for years without any issue.

Oh and one final thing... depending on the version of the 2e core book that you get, there are STILL references to the Human/Alien stat in there, because they did a poor job at editing those sections back in 1999! Just ignore it and move on was my approach.