Do slimes need some floor to spawn in their spawn chunks, or can they spawn midair if I remove all the stone?
The Wiki is inconclusive, the notes marked with  and [Verify]. A discussion from '11 notes they can spawn in solid rock (and die immediately) just fine.
My rough plan for an automatic slime farm is: dig a wide hole to bedrock at slime-spawning chunk using TNT-loaded dispensers, funnel them to a deep pond using water, cover the floor of the pond with hoppers. Slimes would spawn in midair above, fall, drown, and the hoppers would then collect the slime balls. With vertical walls and all side corridors (and surface) well lit, no other mobs should spawn.
That's the plan. Now the question is: will it work? Or do I have to build a bunch of spawn floors for the slimes and light them up so that nothing else spawns on them?