The three guard units that possess Slothjemia’s highly coveted dragonriders assign the riders based on the age and size of the dragon. Hatchlings are not ridden except by smaller races such as goblins, xvarts, gnomes, and kobolds to get the dragons used to carrying people on their backs. After this age category the dragons can easily carry one rider per age category. So a “Very Young” dragon can carry one fully equipped rider, a “Young” dragon can carry two, and so forth. The oldest and biggest dragons are capable of hauling around an entire squad of guards that are themselves trained to fight both on the back of the dragon as well as on the ground in defense of it or in concert as part of a coordinated attack.
An important thing to remember is that these creatures are not standard dragons, having undergone a series of magical and surgical modifications to rob them of their intelligence, cunning, and spell-casting abilities. It would be nothing short of ludicrous for there to be fully capable dragons willing to submit to being ridden about like fancy winged horses. Black dragons in the wild are notoriously cantankerous and very loathe to ally with anyone for any period of time, and even when they do they tend to be backstabbing terrors when it suits them.
However there is one other important thing to bear in mind; even with severe mental hobbling these dragons are capable of doing extreme physical damage with their breath weapons, claws, bites, kicks and wing buffets. Doubtful that a black dragon’s spells ever did a damn bit of good anyway considering their natural intelligence hovers just above that of an overripe cantaloupe. But the riders assigned to these beasts manage to keep them under control with relative ease thanks to decades if not centuries of training. Be prepared for each dragon to carry more than one rider, though, and they are every bit as ready to fight as the dragon is. And all of them have an equal chance of devouring your corpse afterwards.
The first and primary rider is always a warrior trained in melee and aerial combat. They are equipped with extraordinarily long lances to enable the rider to strike at both airborne threats and targets on the ground that the dragon might be swooping over. They also have a variety of other melee weapons at their disposal as well as shields. They are given the most comprehensive full plate armor available and they nearly always have some sort of enchantment on their equipment to make it easier to move in for maximum maneuverability.
The second rider will be similarly equipped but instead of a lance and shield they will have either a heavy crossbow or short bow and several quivers of arrows for various types of attacks. Third riders will usually be a cleric or chaplain, but they might also be paladins or rangers suitably equipped for ranged and close-quarter combat.
Fourth riders are spell-casters with an occasional psionicist thrown in for good measure. From here on out the riders alternate between melee warriors (fifth and seventh) and ranged missile warriors or rogues (sixth and eighth). The overall point is to get force where it is needed and in as sizeable quantities as can be managed. The investment in training, arms and equipment, and salaries for qualified personnel added to the cost of raising, training, and altering the dragons themselves make Slothjemia’s dragonriders the most expensive units to be deployed anywhere in the continent of Partum. Commanders and the Queen herself are therefore reluctant to commit them to battles unless doing so has a high probability of success because losing even one dragon means a substantial financial and tactical setback. The flipside of this is that enemies of Slothjemia can tell from the presence of dragonriders on a battlefield that the Slothjemians believe they have an overwhelming superiority, and this can have a detrimental effect on those that oppose the Empire.
One final note on the dragonriders themselves. When grounded or separated from their units, personnel designated as dragonriders function as one full rank higher than their insignia denotes. These brevet ranks are applied if the dragonrider is trying to rally soldiers to their aid or for any other short-term battlefield function. It would not apply if the dragonrider is ordering a retreat or trying to usurp overall command from a regular officer already in place for the unit trying to be controlled by the dragonrider. The respect earned from riding huge monsters into war only gets a person so far, you know.