I was just reading the Leviathan Codex entry here on Wikia and something caught me:
"Their current world is unlikely their homeworld, so it is surmised that the Leviathans mastered technology and spaceflight despite their immense size and aquatic nature. Their ability to communicate with and mentally dominate land-based sapients would have been a necessary step, and so whatever homeworld the Leviathans hail from would have included a thrall species whose civilization served their purposes."
I'm sure this has been proposed before, but I think the Keepers are the Leviathans' initial thrall race.
The following quotes come from the Keeper wiki article:
"According to Vigil, the keepers were likely once a race that was either created, conquered, or indoctrinated by the Reapers, possibly even the first race they enslaved."
"Chorban has discovered that the keepers are engineered and are designed to react to something like a signal which occurs every 50,000 years or so. In a message to Shepard, he states his theory that 'they were engineered millions of years ago... by the same people who made Sovereign!'"
The following quotes come from the Leviathan wiki article:
"The Leviathans possess the natural ability to influence the behaviors of lesser-minded organisms to the point of complete physical and mental control, an effect that is similar to indoctrination. They can use this ability to communicate with other beings in a manner similar to telepathy, as well as to turn them into thralls."
"The early history of the Leviathans is completely unknown, but it is speculated by Task Force Aurora that they evolved on an unknown planet that also produced at least one other sapient species. The Leviathans developed the ability to communicate with and dominate land-based sapient species to serve their purposes."
Although the Leviathans enthralled an unknown but likely large number of species, whom would the Leviathans trust most with the maintenance of the Citadel? Their first thrall species seems the most likely to me. The Leviathans created the Intelligence, which is housed in the Citadel, so I feel like the Leviathans would have its thralls maintain the station without their direct input, being just easier to manage, and the first thralls would be most loyal to fulfill the task.
I'm aware that this does not matter at all, but I thought it was an interesting theory.