Personally, if something like this were to be implemented, I think a better solution would be to allow the player to convert an officer from 'regular' to 'mercenary' when their roster is full; I'd like to at least have the opportunity to look a newly-rescued officer over and decide if I want to keep them before they go into the temp-contractor pool.
what about the level 7s?
well to put it frankly, the mercenaries and officers from derelict are anomalies in the system, they are not affected by your skills. (especially those that raise level and number of elite skill). that means they can have more level than any of your officer could ever get (7 instead of 6) and have more elite skills than your officer could ever get as well (5 instead of 4).
therefore as the anomalies that they are it would be wierd to consider them equal to the officers you raise yourself.
acquiring officers from derelicts would be an increase in challenge by improving your opportunities but, as they would be dependant to a timer and your possession of storypoints, those opportunities would be limited in time and could force your hand (as mercenaries cost, in story point, increase each time you extend the contract you will not be able to keep them forever unless you cut off other expenses) resulting in a possible backlash when you lost them if you built your strategy around them.
having those officer from derelict as mercenaries is a necessity to improve the game challenge and balance.
The main problem I see with this line of argument is that the officers you get from derelicts aren't going to be tailor-made for what you have or want whereas officers you train up yourself can be, so unless you're building your fleet around the officers you find the level 7s often aren't particularly better than officers you hired at level 1 or 2 and trained all the way up - and even if you are building your fleet around the officers you find, it's possible to roll some really mediocre or niche skill combos on one or more of the handful of level 7s you can rescue, and you might still want to spend a story point to adjust elite skills and personality.
Additionally, my feeling is that mercenary officers generally aren't worth their costs outside of wolfpack and maybe derelict operations builds, because nine to eleven officered ships (including flagship) is often pretty close to the practical limit of what you can actually use, especially below max battle size or if you want to field capital ships. Rescued officers, or at least the level 7s, being mercenaries by default would in my opinion remove a lot of their potential value and take them from being something that's kind of neat to being more or less expensive trash that I want to get rid of as soon as I find it, as is already the case for rescued colony administrators.
