I would highly suggest that you take the Platies back (I can't make you though and I'm not trying). Platies are very active and will probably not last long in a 3.5 gallon (especially males). Usually all pet store workers (including the manager) don't know what they're talking about when it comes to fish. Also, you could have easily treated your Betta in the 3.5. In a bowl she's more prone to stress and sickness because of high ammonia, and the fact that all they'll do is eat, sleep and poop. Most employees will say anything to make a quick buck because they get paid extra if they make a big sale. Someone at Petco once said that it was fine to have 2 Goldfish, 1 fancy, 1 commet, in a 1 gallon fish bowl (this is like the worst ever myth that was ever created). The Goldfish died in a matter of days. As a beginner, you're going to make mistakes (heaven knows I did!), the important thing is that you learn from them, and move on to a higher quality life for the fish!
My biggest dream is to someday have Water Sprite!