Maybe it was tweaked because the modpack maker knew that you could not obtain slime grass and you could obtain a slimy sapling from a Reward Bag?
What soil something grows on, unless a config option exists, is beyond the scope of mod packs. An example is agricraft crops sticks for having a config. Otherwise it is trying to allow saplings to be planted on water.
I know that on AGN and AG2, Slimy Saplings will grow on normal dirt and grass. Are you sure its not a bug, but a feature instead?
On the last server we had the floating islands in worldgen. Not sure whether they got removed in recent updates or whether we just have a bad world seed.
There shouldn't be floating islands in the current regrowth, that was a big during world gen in the previous version of regrowth