I agree completely with MBOU. With your tank as it is, you'll probably find that you don't see much in the way of platy fry, because the majority of them would be eaten within minutes - livebearers are seriously bad parents.
However, you are talking about planting up, and this will give the fry somewhere to hide, and therefore not be eaten. Each female will be pumping out around 30 fry every 4-6 weeks. You wouldn't be able to move them on for at least 3 months, probably longer, assuming you can find somewhere to take them. Thing is, platies are so cheap for the shops to buy from their trusted sources, why would they take them from some moosh who walks in off the street with a bag full of babies?
Not trying to put you off, just trying to help you consider ALL the ramifications.