Go for a mix of the species. in my 30 I have 3 swords and 1 male guppy. I used to have 2 males and 4 females. I think ich medication killed all but my 1 male though. I don't have much experience with Platies.
I think it gives my tank a good mix of medium, and small fish. Its honestly a tossup on if my centerpiece fish are my gold and blue rams or my koi swords