So I looked at things again, I finally found an Asbestos Cave so those clearly generate, I did see the Sulfur Caves too earlier, and Rubber seems to work just fine between Mods.
And as I have mentioned on a GitHub Comment, the Bone to Slime Recipes should not be in the Furnace due to way too many Mods smelting vanilla Items in it into their random Items, which leads to Recipe conflicts.
As for the Creative Tab thing, I was just curious because SOMETHING messed up something, because I made a mistake, which was ending up letting some other Mod just remove NBT from Stuff. But I fixed that on my end by only putting copies of those Stacks into the Creative Tab List.
Anyways, this particular Topic can be marked as solved, because I did pretty much everything that was mentioned here. ^^