Angles matter
The listener needs meaningful angular separation between the base layer and the overhead layer. Speakers directly above the surrounds can collapse that separation.
Aim at the listening area
High-frequency coverage narrows in many loudspeakers. A ceiling speaker or wedge should be chosen so its useful coverage reaches the intended seats rather than simply pointing straight down.
Four heights vs six
Four heights can produce an excellent front/rear overhead field. Six heights become useful in longer rooms where a middle pair creates additional spatial resolution.
Ceiling height changes geometry
A low ceiling can create aggressive near-field level differences. A high ceiling increases distance and may demand more sensitivity/output.
Consistency beats symmetry
Architectural symmetry is nice, but acoustical angles to the seats are the priority.
Apply the principle to your room.
Send the room dimensions, seating plan and proposed system.
Networked in-ceiling speakers are now a real option
Genelec’s 4435A is an unusual example: an active in-ceiling loudspeaker that receives Dante/AES67 audio and PoE power over Ethernet. That does not change Atmos geometry, but it changes cabling, amplification and service access dramatically.