OK... Well, this all really depends upon setting, and whether you are going with the fast and loose 23rd century or slightly stricter 24th century.
Back in Kirks time, it seems that the First Officer could also be a serving department head, whic would keep the occupied in situations, but also give them input into tactics and orders (ewspecially if the captain gets injured).
I say 'could' because Spock was both first officer and Science, just as the demoted Decker in the Motion picture was also both (until Spock returned)... But in the Pilot, No1 seemed to just be Exec, as did Chekov aboard the Reliant...
But in Picards time the roile of 1st officer was definately a seperate role, and effectively a back-up catptain, whose job it was to lead away teams, or to fill any vacant or support bridge positions during red alert and to ensure that the captain was advised of ALL options...
I also prefer the captain remain an NPC, at least until a player has consistanly proven their ability to take-charge in character, in which case let them have their promotion...
As for the 2 tactical officers, split Tactical and Security? Let there be a red shirted Tac officer (I see this as a Command post) on the bridge as well as security chief who also gets all the juicy away teams... When both are present, the senior officer will depend upon circumstance and whether the situation is Tacitcal or Security.
Altermnatively, if you find your players are each after the various roles... Get them to build their characters and then YOU as NPC captain can then assign them. A senior rank would be chief... But perhaps that other Operations officer might also suit Helm or Engineering? Perhaps a transfer to another department but do it as a plot device led by your NPC captain? Using this NPC status to your own advantage, hell, you can even invole the players of the XO and Councilor, make them part of the decision and get them involved?
DanG/Darth Gurden
The Voice of Reason and Sith Lord
“Putting the FUNK! back into Dysfunctional!”
Coming soon. The USS Ganymede NCC-80107
"Ad astrae per scientia" (To the stars through knowledge)