Congratualtions on your new beardie! I don't think it really matters what kind of vitamins you feed. Personally, I use the T-rex one because that's all they carry at my local pet store. If you're worried about the prey hiding in the hide box, I would just remove it when you went to feed your little guy the crickets and then put it back in when he was done feeding. I never put a hidebox in with mine when they were that little because they hid under the paper towels anyways.
It is true to wait until they're older to feed them mealworms. The reason why is because mealworms contain a high level of chitin which is hard for them to digest, which can lead to them becoming impacted. I don't know what discoids are, so I can't help you there. What are you using for substrate? Since it's a baby, I would use either paper towels or newspaper. Also, where are you getting your little guy from? And lastly, you'll definately have to post some pics. :scatter: