Tossing Dispossessed in with Fyreslayers doesn't work the same way it doesn't work with Kharadrons.
Back in Ye Olde Worlde, Slayers and the more 'out there' Engineer units were just small parts of the army (Heroes, Special or Rare) and aesthetically designed to fit in with the rest of the range, both in terms of rules and aesthetics.
Fyreslayers and Kharadron however both have very distinct rules, lore and aesthetics as they're intended to be standalone armies. Put the Dwarf Gyrocopter next to any of the Kharadron airships and you can see there's a distinctive, deliberate difference between them. Hell, put Gotrek next to a Fyreslayer and he looks 'off'.