I'd personally prefer a tag, but it'd be nice if both slots in a Reactor could accept different items based on what was tagged/configured.

Having it apply to at least Carbon would be good imo.

Ideas for the tag's name:
Carbon Slot: carbon
Redstone Slot: stabilizer (also changes the hover label for redstone)
Uraninite Slot: fuel

Adding a section to the config to add more solid coolants would be great too. There's one for Thermo Heat Sources, so it's a bit odd there isn't one for Coolants too.

ah, didn't know it was different. Thanks for telling me.

would recommend reporting here (because this will most likely not be locked at here): https://github.com/Technici4n/Powah/issues
(this is the maintened port of powah for version 1.18+)