15g tank, right? i'd go with another harlequin and 3 more male guppies. that'd give you:
Male Guppy = 1.5" x 5 = 7.5"
Harlequin = 1.5" x 4 = 6"
Platy = 3" x 1 = 3"
All together that would give you 16.5" of fish. Unless you want to buy a bigger filter, add plants and clean the tank twice a week, don't stock any more fish. The guideline for the most fish with least effort (adequate filter, no live plants, change water 20% once a week) is 1" of adult fish per 1 gallon of water. You will already be overstocked as it is.
now, this is why i think you should have 5 male guppies, 4 harlequins, only one platy, and NO MORE FISH:
harlequins are tetras and are schooling fish. they are much happier and easier to keep in larger groups. generally the reccommendation is 5 or more, but you don't really have room.
male guppies are really nippy and will chew each other's tails to shreds if not kept in larger groups. 5 male guppies should (in theory) do ok and not munch on each other too much.
female guppies will do just fine in a pair. if you were intending to keep 2 female guppies, don't bother getting any more guppies and get more harlequins.
don't bother getting more platys. one platy is perfecty ok in a non-aggressive community. it's much more important that you get more harlequins (or male guppies if necessary) that it is you get platys.