The fun thing is that there are rules for all of those in the expanded campaign ops/merc handbooks. Since not enough admin staff will lower your merc rating, and not enough technicians will lower your merc rating, I would hire the one that raised my rating the most. AKA, if I dont have a mechanic, but have vehicles, well you need to have a mechanic period, so you must hire this mechanic. So without further info on exactly how the merc unit is staff deficient in the actual rules, my answer is the cop out 'the one the unit is short the most, as determined by the staffing rules of the game'.
Now, if the unit is fully staffed and this hiring will not improve the merc rating, if using the training rules from ATB the training officer needs to be out leading a lance, so the as worded 'non-pilot' solaris guy is kinda useless for granting XP. Also, unless you have a lot of green recruits, training wont help. But if you do have green recruits, and the training officer can still pilot and lead a lance, well bring all your green recruits up to combat regular is a must have. Same with the doctor, an elite doctor is nice only if you dont have one already AND you are using the medical rules. If you arnt using the advanced rules, then its kinda just a fluff pick, and you are better off with someone that does something tangible for the unit in game.