3-4 inces snout to vent? Your chams should easily take 1/4 inch crickets if they are indeed that size. If you dust the crickets properly, they will be almost a chalky white color. You shouldn't have to use a lid. Your veileds should be able to see them as they crawl up the cage walls. One thing about the reptariums though, the crickets will eventually chew holes through the upper corners, and zipper strings will catch. I prefer the screen cages.
As far as keeping the two together- I have succesfully raised chameleons up in the same cage(M&Fpairs). However, I must caution you that the pair you purchased were probably from the same clutch. Not meaning to knock LLL,( I love their screen cages), but big companies like that tend to buy entire clutches from breeders, and there is a good chance that your pair is bro & sis. If your plans were to eventually breed these, I'd look for a different pair. Put the new guy with the female you already have, and the new girl with your previously purchased guy.
Hope this helps,
Cricket