SU Fan Theories. This is purely speculation with little or no evidence in the series.
My theory will be based on this one.
To Perfect :
The real reason Homeworld dislikes fusion. The Diamonds may have fused in the past. It may be that reason is way they look down on fusion.
When they fused (or if), their fusion might not have wanted to separate. It may have been necessary for 4 diamonds to rule homeworld.
I think their current hierarchy may have been the result of a different / similar system. It's not that the diamonds truly dislikes individualities but by using their current system that's in place they somewhat prevent fusions from happening. If everyone has a role, then everyone has a place and if everything is in place. There is less of a chance or two gems bonding enough to be able to fuse.
The diamonds fusion might have been toxic to their personalities. We've seen how fusion can be a bad thing in the series. The diamonds might have experienced it first