Hostile mobs spawning rules on PS4?

I found a skeleton monster spawner and build a xp trap around it.
It’s the typical trap described in all the YouTube videos:

Source: YouTube Tutorial by KaspersMC (this picture doesn’t show the final trap)

The trap works and I wanted to extend it, so that also other hostile mobs get caught. Therefore I added two more rooms that look exactly like this one but without water (because I read something about hostile mobs can’t spawn – besides by a monster spawner – on flowing water? Is that true?).

Rooms:

• completely dark
• all cobblestone
• 9x9x3 (LxWxH, at some points even a height of 4 blocks)
• tested with and without water

Problem: Other mobs (but skeletons from the mob spawner) don’t spawn. Neither in the two new rooms nor in the first one.

I already tried to go more than 24 blocks away, wait some time and then come back to check the rooms but they were still empty!

I checked during day and night.

Questions:

1. Can other hostile mobs spawn near mob spawners in general?
2. Can hostile mobs spawn on floating water?

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Platform: PS4 Pro
Version: 1.74
Mode: Survival, single-player
Difficulty: Normal