I can see it going either way, Mollys might be fine but it might see the Platys as food. If you get a small BP you should be fine but there is always the chance.
I think that the bp will eventually eat them when it get bigger. It will have NO problem with the platys. If you decide to do it make sure u have a divider! But, like chris said it could go either way...
A long time ago I had a Bp that was completely friendly, in with tetras and corys. Although, they can also be mean... Get it small and grow it out with the platys, and it could work