If you can afford a leech who isn't the sync in your party, then you can afford to not have a 6th person at all.
When I do it, if we can only get three SMNs, a COR, and a sync, we stop there, and just have the sync toss up a job at level 14 instead of 12, and pop exp bands.
That being said, never offer to pay for something that you can probably get for free. It pays to be friends with Summoners for this sort of thing.
It also pays to level your **** jobs so that your skill stays capped and you learn how to play the job, even if it's only to level 37.