Skrall have consistently been referred to as members of the Rock Tribe, which they share with Agori of rock like Atakus. Does this mean that they are a subset of the Glatorian species, or are they a species all their own?
Tribe membership doesn’t delineate species; Gresh is of the Jungle tribe as a Glatorian, as is the Agori Tarduk as a civilian. Not only are tribes trans-species coalitions, but Skrall are on numerous occasions specified to be a different species to the Glatorian. You can see this on BS01’s page on sapient species
No, the Skrall are not of the Glatorian species. However, there are also no “subsets” of the Glatorian species. There are no genetic differences between a Glatorian-species of the Jungle Tribe vs. Water Tribe.
There might be regional differences, but that was never really covered in-story.
Now I’m wondering if any members of the Skrall species ever joined another Tribe.
The Rock Tribe probably wouldn’t let in any non-Skrall (at least not pre-Battle of Bara Magna), but I could see, for example, the Jungle Tribe welcoming a Skrall (with some suspicion).