Yeesh...I don't even know what to say to that advice.
Bettas are tropical fish and come from a warm climate. That means they need a temp of (at the very least) 75 degrees. But the higher, the better.
1.5 gallons is okay for a temporary
tank, but you will have to do a lot of water changes, so it's not really worth it imo. We recommend at least 2.5 gallons for your fish. Just because they can survive in a smaller place, doesn't always mean it's better. A lot of animals can survive in tight spaces, but again, doesn't mean you should keep them like that.
With a 2.5 gallon tank that's uncycled, you'd need to do at least 2 weekly water changes. I do one 50% and one 100%, but some people say you'll need to do two 100% changes.
Most people feed their betta twice a day. I feed mine 2 pellets during the day, and 2 pellets at night. Their stomach is about the same size as their eye, so that should give you a good idea of how much to feed.
And if you have any questions, come to us! We can give you the best advice possible, which normally can't be said about pet store employees. Unfortunately, they usually don't know what they're talking about.