Nice work on the viv and it should make a very happy home for them.

Just buy crickets that are no longer than the distance between the frogs eyes.

Use a glass bowl (like a cereal bowl) for feeding. The crickets will be unable (most of the time) from escaping and if the bowl is sunk into the substrate the loose cricket will fall back in eventually.

When you mentioned vitamins, did you pick up calcium? Calcium should be dusted on the crickets at least three times a week while they are growing. You may also want to get the humidity down to about 70 percent since 90 is pretty high.