It boils down to personal taste and experimentation of what will work and what won't.
For me,keeping all the heterospilus together would be a definite. (as long as they all keep getting on)
I would also want a decent sized shoal of swordtails just to add more activity and another dynamic to the tank.They will also provide more action higher up the water column.
If you want swordtails you may need drop the JD as it may eat them.
Personally I would do the heterospilus, swordtails and add a group of meeki and leave it at that.
If you do stock it this way I for one would be quite jealous. I love hetros and really miss keeping thorichthys.
Regarding synspilum /melanura, once fully grown they are big and I felt my 7x2x2 was a little small for my full grown male. I think with the heterospilus it would be a little cramped.
Good luck.
The thought of my JD eating the live bearers did cross my mind. He's very wimpy for a JD but even still. I was also thinking Meeki as I think their reds would contrast the heterospilus nicely.
Say I were to keep all six heterospilus, what size group of Meeki do you reckon would be good?