So I am a well-read Bukkit programmer coming to Sponge, and I love what I’ve seen so far. By far, the most common thing people yell at me about is coding in Java 8 occasionally, since their servers use Java 7. And the documentation for Sponge says that it supports 6-8. But whenever I try to compile …

Sponge only supports Java 8 now. As Minecraft has made the move to 8, Sponge will follow. https://docs.spongepowered.org/en/server/getting-started/jre.html