I have a 2ft tank its about 13.6g and about 55-65ltrs (the box says 63 but apparantly the sizes dont measure up) I have an overstocked tank but I think you are ok for the moment. If you get rid of the platy fry you could probably put your bettas in there - as long as the bettas you alreayd have are female!
i dont kno how much gals it is but i can tell u the dimensions:
12 inch X 14 inch X 24 inch
as for the fry Im ether gonna sell them or just let nature take its place and let the bigger fish eat them
the bettas i got are 2 males ( in a divider ) 1 female and im going to get more female bettas, maybe one more male to put in the divider