Indeed, specifically Outsiders are in-text referred to as coming from the the Nether World. There is definitely a big link. Jim will of course have his own spin...but even in Hellboy there are links between, and in Warcraft, and D&D between the Celtic/Norse/Greek/Faerie/Elves and Demon. And JB uses all those, plus regular mythology and a few other fictional works.
It is just we know so little about them in-world.
Fact 1: They were related to the Fae and the Jotuns. The married, fought and were eventually driven back to the sea.
Fact 2: The currently are a nation of outcasts, not just the original Formor, but dark gods and spirits united together.
Fact 3: The Cantrev Lords look like frogs and have powerful, water based magics. (This further shows Harry's understanding of Magic is seriously limited).
Fact 4: Their servitors (turtlenecks) steal magical talents for an unknown purpose. They use both modern and ancient and arcane technology. They are sort of like magical cyborgs (but instead of machine parts, they have weird animal and monster parts).
Fact 5: The Red Court was somewhat keeping them at bay, or at least less active. When the Red Court fell, the Formor quickly and fiercely took its place.
Fact 6: One of their Cantrev Lords is Lord Omogh (who I can find not much info on), and another is Mags who was defeated by Marcone.
Fact 7: Their leader appears to be King Corb, who's court is located in Lake Michigan (I mean come on JB, change it up a little, we get you like Chicago but not everything of significance has to be there, there is an entire PLANET outside of America, let alone Chicago).
Fact 8: They created the beasts used by the Denarians, the hybrid monsters. This suggests dealings between the two groups.
Fact 9: The have had dealings with the Corpsetaker...and Evil Bob. This suggests a link to the Disciples of Kemmler and perhaps even the Black Council/Circle.