Platies and Danios will get along famously. The danios will chase eachother tirelessly across the top of the tank and the platies will spend most of their time in the middle layers away from the hustle and bustle of the danios.
I wouldn't be too worried about the danios making tons of fry, they eat a lot of their own eggs and, even if some make it, the fry wouldn't stand a chance.