Hang on before we make any justifications, do you know the dimensions of the tank? Do you also know the general hardness?
As
@Sgooosh said above mollys and platies like harder water usually in the 250ppm range whilst your corys prefer water around 100ppm. ppm is the measurement for what the GH is. GH can be found by going to your local water companys website, putting in your address or postcode and seeing how hard your water is. Sometimes it will come up in German degrees which will have the number and dh on the end. Also is this pygmy catfish small and have a black line down it's flank? If it does it is Corydoras pygmaeus and need groups of 10+. I don't think your tank can support that much but 6-8 could work.