I have had a lot of platy fry in my tank over the last year or so. Aslong as you have some good hiding places for them they should be fine. Many of them probably won't survive. But personally since I got my platys last year i've had a good survival rate - and I now actually have too many!! I have some larger fish in my tank which definitely have eaten a few.
I also have a lot of plants in my tank and when I look closely I can see a lot of the fry hiding in amongst this. Bettas are quite aggressive so he probably will eat them. You can always buy isolation fish boxes for fry, I have one and it worked really well for my Cory fry. (super cheap on amazon)