New background and models often draws inspiration from old stuff and sometimes is faithful to what was before, sometimes not.

A Defiler is a daemon trapped inside a mechanical body, so it's a demon engine just as much as a Slaughterer is (just one that can be allied to any power even bland undivided). A Slaughterer is smaller though and more akin to a Khornate demon dreadnought than the larger defiler (both epic models sadly are somewhat oversized, but hey look cool). A Soul Grinder is a variant Defiler in the W40k Daemon army list that is 'unbound' and visibly possessed by a powerful demon. The new Plague Hulk from Forgworld looks to me kind of like a variant Soul Grinder.  

You can categorise things if you like though chaos doesn't fit neatly into categories, nor should it. A Silver Tower is a lot smaller than the others in your greater list, I'd put it in the lesser category and assume Tzeentch has no greater one. A Hellstrider is a Slaaneshi Knight and not overly demonic or beastlike.

To add to the confusion have you encountered the re-imagined Brass Scorpion yet? That is something that has been gradually re-envisaged over time, first with a semi-official defiler conversion in W40k and then later with an official massive war engine sized Brass Scorpion, meaning you now have 3 versions of the same vehicle in different sizes and which have been included in current epic Khorne fanlists as things like 'Greater Brass Scorpion'.