Wow, the pet store people are educated over there:D Most of them are ignorant -___- but they are right! It is best to introduce the tank mates first, so the betta is entering their territory. However-- make sure you betta will get along with the fish, some wont. Make sure you have a back up plan if all fails!
The platies are fine, and its never too late to get the betta, since platies are peaceful fish! Beware, they may nip though. Also, watch out for your snails-- there have been cases of eaten atennaes because the betta!