At United, the only non pilot seniority list instructors are the ground school/systems instructors. All sims/flight training devices or line instruction (IOE) are done by seniority list pilots.
And no pilot instructor got hired as an instructor right off the bat. They got hired as line pilots, who then applied to become an instructor in the training center. I believe they need to have 12 months of line flying before they can apply to the training center.
Most of the sim instructors are First Officers (legacy CAL had some Captains as instructors so they may still be doing that gig) due to the pay cap that is specified in the contract. Standard Captains and LCA's are paid at their respective rate plus an override for conducting whatever training they are doing.
DC