Senior staff maybe just "the Crew" that is usually asigned to the defiant, but realistically, He's a main character, and he's there reguardless of seniority.
I put it as this, speaking from experiance, When I wen in the the military, I chose what I wanted to do ( This was enlisted, but its the same for officers, Pilot, etc.) which was a general "Aviation" that was put in to my contract, When I was at school, they asked us where would we like to be stationed, Need of the Corps came 1st, but they did take that in to account ( I chose west coast.. and go way way west coast... of Hawaii.. Useded up a lifetimes supply of luck on that one!)
So as shown on the show, the upper percentile usually get there choice in assignments on various ships, stations, etc. The rest still choose where they want to go, if available, and a commanding officer's recomendation to take someone would pretty much gurantee them that assignment. With Picard, We've seen him request certain graduates, like that Bajorian from red squad, because he wanted to give her a chance. I'm pretty sure he takes on what he can, even bottom tier graduates, to see if, given the oportunity, they could shine.
I'm sure there's some nepotism, Maybe Admiral Paris asked his buddy Captain London to ask for his son to be on some top of the line ship.