No, you really can't overfeed a juvie. Just as long as their crickets are smaller than the space between their eyes, they can be fed as many crickets as they'll eat in a 10-15 minute time period.
There are lots of greens and veggies that bearded dragons can eat. Some greens are: kale, turnip, mustard, collard, dandelion. Some veggies are: any squashes, yams. They can also have some fruits, as long as they're not citrus. Mine love mango, papaya, kiwi, blackberries, raspberries, just to name a few.
There is a very comprehensive list of foods beardies eat, how often they can eat them, and that food's nutritional content at
www.beautifuldragons.com
Do you have a natural foods store in your area? They generally have a pretty decent selection of greens and veggies.