ruffmeister said:
its like cats and dogs breeding they might mate but their young will never survice because of the shape and size difference same as with platys and mollies they are just not able to carry til birth
I'm sorry, but that is the most ridiculous statement I've ever seen.
cats and dogs will never, ever mate - they are completely different species.
even if they would mate, they wouldn't produce dead young, fertilisation would not occur.
It's nothing to do with shape and size difference, it's to do with genetic difference.
Different sized animals can interbreed with no problem as long as they are similar enough genetically - e.g. large horse x pony, great dane x chihuahua (ok that one isn't likely but it is theoretically possible)
platies and mollies are genetically similar species - they will mate, and fertilisation may occur.
however they are not close enough related to produce viable young (I'm not familiar with the technical genetics of these two fish - it may be possible for them to produce live young, but very unlikely, and the young would most likely be infertile if they did live).