I understand you were given the wrong info at the petstore. They usually don't know much about chameleons. This site is full of experienced keepers and I would take their advice over a pet store employee. The mealworms should be stopped altogether. You can feed crickets, dubia roaches, silkworms, black soldier fly larvae, also known as calciworms, and as treats you can feed hornworms, superworms, and waxworms. As for cage recommendation the reptibreeze xl is relatively cheap and is the required measurement of 2x2x4. You should be feeding about 10 to 15 crickets if that is all you are going to be feeding for now. At this age they eat a lot.