Mnk is very weak in pts until about 25. Mnk pre-25 imo is a mediocre DD w/ some defensive abilities so /war and help your tank out when pt gets into trouble. Since you won't be pumping out damage like you will at higher levels, make sure you get some good DD in your pts. Blms and Rngs are the standard DD you want. If you do have to go w/ weaker DD's, try to get good support jobs like brd and rdm.
Honestly though 90% of the time though, you're going to just take whatever you can get, bc after 5 hours of seeking or trying to start a pt, you don't care what job they are and you don't care how gimpy they are. So just try to be versatile as a player and know your job so no matter what situation you're in you can contribute to the pt and make it run well.