Servitor don’t bring anything.
But yes, both (Table Top) Skitarii and Tech Priest don’t make sense as Rejects.
But Skitarii have lesser variants like the Hypaspists.
Hypaspist Skitarii were the most basic augmented infantrymen of the Tech-Guard and were armed with cybernetically hard-linked Lasguns similar to those used by the soldiers of the Astra Militarum. Hypaspists were generally more powerful and skilled soldiers than standard Guardsmen as a result of the cybernetic enhancements that made them stronger and more accurate than normal humans and the psychosurgery they had undergone to suppress their emotional reactions and make them immune to panic or fear. However, this enhancement also made Hypaspists less independent and able to think for themselves in combat.
But in any case the way to make the Admech themed archetype work with a Reject is to neither pick a Skitarii, nor is it to pick a Servitor or an Adsecularii.
Instead take Tech Menials, those civilians that are either born on Forge Worlds or as part of the local Admech communities (Tech Guilds and such), who when they’ve done “crimes” are then sent either to specialised
Admech Prisons, or just to Servitorisation stations. Where we (or more exactly the Inquisition) instead manage to pick them off before they arrive to destination.
This let them introduce them as Reject, while giving them various origins.