Ok then:
WHEN it is cycled and
WHEN you have rehomed the flying fox
you should add more rasboras. Nothing else! That is the extent of the capacity of a cycled 15 gallons. You might just get away with another guppy or platy. You should NOT be looking into adding another species, any other species, but should be looking after the ones you've got as well as you possibly can. Rasboras need to school and livebearers (guppies and platys) too, though not requiring a regular school, are happier with a couple of companions of their own species.
Let's do the maths. For a new tank, in the hands of a beginner, with no extra filtration, the rough rule of thumb is 1 inch of fish/gallon of water.
Platys are 2 inches long. Female guppies also 2 inches, males closer to 1 inch. Harlequin rasboras about 1 3/4 inch. Flying foxes I believe can get up to 6 inches (note that a 15 gallon should not house any fish longer than 3 inches with the possible exception of a khuulie loach). So we have:
1 platy=2
2 guppies= 2-4
6 harlequin rasboras (for a healthy school after cycling)= 8 1/2
1 flying fox (to be rehomed)= 6
----------------------------------------------------
18 1/2- 20 1/2= overstocked
rehome the fox and add a guppy or platy (later, much later!) and you are safe at around 15 inches.
Before you add any fish, make sure you have read and understood about such things as gender ratio (for livebearers) etc.
In the meantime, keep testing the water- and learn as much as you can about the fish you have. We are always happy to answer questions about the care of guppies, platys and rasboras. When you feel like aquarium shopping, instead of buying another fish- why don't you do what I do: treat yourself to some plants? Or something nice for them to eat? Or a tank ornament?