Unfortunately I don't really think this is a good idea. Platies like much different water conditions than bettas do. Bettas like acidic water with no salinity. Platies prefer harder water with higher salinity. Platies are also aemi-aggressive, and because bettas are a slow moving species, the betta could be harmed. Also, you picked a very vibrant color of platy--the male betta may confuse this for another betta and attack it. Keep in mind that some bettas never learn to tolerate tank mates and must be kept singly. Each fish is an individual, especially where aggressive fish are concerned.
If you're getting a 5 gallon tank, I suggest simply leaving the betta by himself. There's not really enough room for a small shoal of peaceful fish, and depending on your betta, he may not tolerate them anyway.
I do not recommend aquariumfish.net --- it has terrible reviews, with a history of DOA fish due to terrible packaging. They also mess up a lot of orders. I would not buy from them. http://www.liveaquaria.com
is a great resource, I have heard nothing but good things and as usual, foster and smith's prices are great. I also suggest finding a local aquarium society as a source for fish.