Derailleur compatibility?
 

I have a road bike with a triple crankset with 30,42,52 chainrings. I have an 8-speed cassette with 12-25 cogs. I have a Sora long-cage rear derailleur and Sora 8-speed brifters. I need a new rear derailleur. Other than getting a new Sora, which rear derailleurs are compatible with my setup? Is it correct to say that I can buy any brand 9-speed rear derailleur as long as I never use the largest chainring and largest rear cog at the same time?

You Cannot buy any brand. You need to match the pull ratio of the Shimano so you need a Shimano RD. Some others will work but no sense in getting into that right now. You will need a long cage for it's chain wrap capabilities.