If you want DRG and MNK, you need LNC and PGL to 30 and MRD to 15, that's it. I have no idea how useful Mercy Stroke is for DRG or MNK; you might eventually want MRD at 26 to get it, but it's not necessary, and it's certainly not something you have to worry about right now.

Note that once you unlock the jobs, LNC and DRG do not level separately, and MNK and PGL do not level separately. A job is a piece of equipment (a soul crystal) that you equip while you are on its base class. Whether you're currently a LNC (DRG soul crystal not equipped) or a DRG (soul crystal equipped), you are actually just leveling LNC.