I am of the opinion that platies and bettas really shouldn't be mixed--platies are semi aggressive and can bully a betta, they also prefer very different water conditions than bettas do since they like harder water with some salinity, whereas bettas thrive in softer water with no salinity. Their bright color also may confuse the betta and cause it to attack the platy, thinking it is another male.
I think you should go with a school of pygmy cories--these fish are social and should be kept in a large group of at least 6-8 individuals. ADFs are also a social species, and should be kept in small groups of 3 or more. Personally, I would add the peaceful inhabitants first--shrimp, then cories, then ADFs, then the betta. The betta is less likely to attack the tank's inhabitants if he doesn't feel that the tank is his territory.
Keep in mind that if you get shrimp, they are very likely to end up as betta food eventually--if that bugs you, you might want to skip it. If it does not, I suggest getting a cheaper variety, such as ghost shrimp or red cherry shrimp.
You should research and understand the Nitrogen Cycle before you put fish in the tank, and read up on how to do a fishless cycle on the tank. This will make your life a lot easier.