Have you ever created your own crew break down of any of the other ship before? If not, maybe you need to start by thinking about what basic functions need to be taken care on the ship, and think about crewing that, with three shifts each. May I suggest you look at
modern ship complements, of a similar sized ship for example, and work your way from there.
Command/Admin
Engineering
Operations (Navigation, Helm, Attached Smaller Vessels (Flight Ops/Shuttlebay), etc.)
Science
Medical (Dental, Psyc)
Security/Tactical
On Smaller Vessels, the same roles exist, but a single person may share the duty of multiple roles, that on a larger ship would have its own division aboard ship.
Maybe a better, more slim, ship orginzation you might want to look at is a merchant ship, which has even fewer personnel aboard.