But as I understand it, Adium can't just use the same plugins as Pidgin, they're very similar but need to be modified a bit.

I ran into this ~ 6 months ago when I was trying to make Slack work in Adium. The plugin for Slack was outdated. There was a newer libpurple Slack plugin, but it wasn't compiled for Adium. I attempted to recompile it, but I couldn't seem to manage it, all of the tutorials on how to do it were super confusing...

(If anyone knows how to make Slack work in Adium in 2021, please let me know!)