Your tank is already at full capacity and has too many fish in too soon,you cannot add anything else to the tank The tank is also a little too small for the mollies and swordtails which need to be kept in at least 10 gallon tanks i also would not recomend keeping male and female livebearers in a 5 gallon as it will soon be over run with fry which will over load the filter.
Sorry to be the bearer of bad news but thats just how it is.