Golfish and Platies are not compatible for other reasons. Goldfish are coldwater fish, while platies are TROPICAL fish, meaning they need warmer water, best kept at around 76-79 degrees. Pretty mich, that means someone in your tank is prettyunhappy right now.. depending on what temp your tanks is at.
Also, it is not stupid for the goldfish to eat the babies.. that is natural for them to do so. Fish that fit it it's mouth is food...