The reason they were an individual is it fits the rest of the way mounted troops are done in the game. They are all individual troops. The bloodhunters, akin to something like the goblin Worg riders, were a unit, so your model would them, yet the individual knights are a bit tougher, at least the lance damage initially is high.
If they were in a unit, they'd need to be more like mounted Firstborn Knights on mounts, and like the bloodhunters they'd be a costly unit. 2 to 4 is a good size for them. So you can have a unit of 2 if you wish.
Same goes for the Mounted Axemen. Personally, you could keep the mounted axemen (or mounted crusader) an individual and the knights into a unit structure.
I made the Tusked Raiders into a unit of 2 to 3, and the Horned Raider I kept an individual but they could easily have been made into the same 2 to 3 structure. I just happened to own one of them, and 3 of the others, yet I played with it, and it seems just fine.
I'd assume the concept can be expanded: Sentinels, Mounted Kutara, and the Eleven Mounted Knight (the old warkiller). Honestly, some, like the true Warkiller are designed to be individuals. I'd say play test this, and it might require modified stats so the mounted units are slightly less buff or LD heavy ie you'd never have 4 skyriders or warkillers with their high LD. That means lower their LD and other key stats, lower the cost appropriately.