Thanks! I am still working on plugins, but I'm generally kept fairly busy by my own ARK servers and work.
For the "across clusters" question, are you using a cross-server chat plugin and want the icons to go over that? I have looked into that before, but I can't see any way to make it work from this plugin alone. Sadly, it really needs explicit support built into the cross-server plugin to work, and keeping this plugin fairly specific to just icons allows people to use whichever cross-server chat plugin they like. Though, I would be open to modifying this plugin to help cross-server chat plugins support this if any of their authors are interested.
If you're not using a cross-server chat plugin, copying the config file to all the servers on the cluster should work for the steam id and group lists, but tribe ids differ per server and would need to be specific to each server.
I don't have any specific experience with using modded icons, but I'd assume it would just be finding the correct path to the icons (it may have "/Mods/" in the path).
Are you getting any errors from the plugin (something like error 1114)? I would expect an error from that config.json since line 14 (the second steam id) has a trailing comma and is the last entry in the list. The fairly strict JSON parser that is in use considers this invalid as it expects another item to follow and it won't find one.