In some fanfics, I like to split the role of Security Chief and Tactical Officer as it makes sense--both are going to be the most experienced and skilled officer in his/her/its field, so on a mission where you need that experience both on a planet and onboard the ship you would need two officers to fill the posts to keep the ship fighting fit and safe. On a ship with just 14 people it seems pretty ridiculous to have Tactical as a stand alone role.
I actually thought up a crew breakdown and rotation for an Archer-Class ship, though slipped in an extra shipmate to bring it up to 15, thus having five per shift:
The three senior most officers onboard, designated the Watch Officer for each shift. The First and Second Officers would specialise in a certain field (eg Communications, Tactical, etc), so they would man a console during their normal shift or when on alert status.
An additional gold-clad crewmember on each shift, usually assigned to Bridge duty as Helmsman/Navigator, but also a rated Ordnance Specialist. During alert status, one would be on the Bridge, whilst the others would see to shipboard weaponry, assist with security and/or emergency protocols.
Each shift would have a single crewmember in blue, cross-trained in a scientific field whilst also a certified corpsman/medtech/nurse. The senior most of these would be ships Doctor. During emergencies, one would man the Bridge, whilst the others prepped Sickbay.
An Engineer would be present on each shift, monitoring ships systems and conducting general repairs. All three would be at work in the Engine Room when needed.
A Security Guard/Field Scout would be attached to all three shifts. During normal operations they would pretty much be a dogsbody, doing all the little jobs here and there that were needed, as well as all normal security training, etc. They would really come into their own when on mission or at red alert.
That was just some thoughts that came to me. There would only be a need for four officers in this arrangement (Captain, Watch Officers and Doctor), though more could be assigned if necessary. On such a small ship as well, perhaps they work a shorter day, for example 18 hours; 6 hours on shift, around 8 hours sleep, giving them 4 hours personal time.
It's one way I like to work things in my mind anyways.