Generally speaking, Soul pet trainers usually train war general level soul pets to the peak monarch level at most. Very few people would spend a lot of resources to strengthen them to the emperor level. Some pickier soul pet trainers might not even strengthen their commander-level soul pets to the emperor level, but instead, look for monarch-level or higher, or purchase emperor-level young pets.
This is mainly because the skill effects of low-ranking soul pets aren't very prominent, making their racial abilities seem more generic. If these pets were to comprehend some abilities, perhaps it would be fine, yet those without comprehension and remain stuck at the war general level, even if strengthened to the emperor level, would still be restricted by other soul pets with powerful racial abilities at the same level.
Of course, there are always exceptions, so soul pet trainers must carefully weigh their options.