In the books, the positions of tactical officer and security chief are separate. But, the security chief can substitute for the tactical officer when that officer is unavailable. Are both members of the senior staff? Can the tactical officer substitute for the security chief?
You could do anything you want. Is it a good idea? Maybe, maybe not.
Jessica served as Tactical and Security in the beginning because they were short handed. She was more qualified as security than she was tactical, however, and she had to learn that position.
She now serves as Lead Tactical officer, and as Chief of Security and Intelligence. However, with the latter two she has help from her staff. Eventually, those roles will all separate, and she'll have to choose one position. (Or rather, one will be assigned to her.) But for now, it works. Most likely, as their other tactical officers get training and experience, she will no longer serve as tactical.
So, it is not a matter of the Security Chief being able to sub for tactical by design, rather its by necessity.
Note that Lieutenant Delaveaga, the Celestia's Tactical Officer does not serve in any other capacity.
Another reason that they are somewhat choosy about tactical officers is that the TO is the one that takes the conn when the captain or XO are not on the bridge. (Either the helmsman or navigator, depending on which of them is higher ranked, is third.)
I see the reason for the Helm/Navigator being third in line (fourth really), is that the pilot will always be busy not crashing the ship, while the Tactical Officer will be more aware of everything else that is happening.
I think the lead flight officer stands fourth in succession of command is because that person (whether the helmsman or navigator) is the next most senior officer after the captain, executive officer, and tactical officer.
However, saying that does bring up a rather interesting point. Unless I'm wrong, Nathan's command of the Aurora has been legal since he assumed it. Yes, the Aurora was shorthanded when she launched to secretly test her jump drive, but when she did Nathan had already been promoted by Captain Roberts to lead helmsman over Cameron. Legally making him fourth in command of the Aurora. So, when both Roberts and Montero were killed in action against the Ta'Akar and in the absence of a tactical officer since one had not been assigned at that point, Nathan legally became captain of the Aurora. All the questions that the characters had about the legality of his command in subsequent episodes were pointless because under EDF regulations Nathan, in his capacity as lead helmsman and fourth in command, would have been required to assume command as soon as Captain Roberts died. Certainly Cameron, with her knowledge of EDF regulations, would have known that.
The lead flight officer isn't necessarily 4th in line for command, he/she is however third in line to take the conn until relieved.
For example, had the Aurora's Chief Engineer survived, Nathan would have relinquished command to him once he reached the bridge.
Two different successions: Conn and Command. Command is strictly by rank of command and combat qualified officers on board. (Hell, the chief of janitorial services could take command, if he/she was command/combat qualified AND was the most senior officer left alive.)
Frontiers saga could have been a whole lot different if the Chief of Janitorial Services survived that first battle with the Takara... Good thing he was called to unblock a drain in the forwards section of the ship / just didn't know the Captain had died...
So, what is the order of Command succession and where did Nathan fall on it when the Aurora left the OAP?
He was the most senior office alive on the entire ship. Everyone else were either Ensigns, enlisted, or civilian contractors.
