My mealies are in a 6qt Sterilite container. For ventilation I have small holes drilled around the top edge. This proved to not be enough for me so I ended up also cutting out an opening on the lid and gluing down some screening.
Their substrate is mostly oats with some whole wheat thrown in, sometimes barley, sometimes bran flakes, or whatever healthier-type cereal I may have on hand. I give them bits of carrot, potato, apple, greens, orange, etc too. (be careful not to over do those or you may end up with mold or bugs).
I used to keep beetles, worms, and pupae separate but I found I ended up with WAY too many so now I just house them all together. This of course depends on the number you need and how they do for you. I have a spare container to make cleaning easier.
Keep old fresh foods cleaned up (though honestly mine eat through what I give pretty quickly). It's easy to see when it's time for a full clean as there'll be much more mealie poo than cereals. I just prepare the spare container and sift through the old one, moving over all the pupae, worms, and beetles. I hang onto the old container for a while to catch any tiny babies that I may have missed with the first sifting once they've gotten a bit bigger.
Good luck! They really are quite easy to raise.
Edit to add: When I feed live to the crabs I just put them in a dish with smooth sides that are high enough the mealies can't crawl out, yet not too high for the crabs to get in and out. I find the little plastic side dishes from KFC (I really don't eat out as much as this sounds
) work quite well. My crabs love them live! Dried too but live even better.
when giving them to the crabs do you just put them in the food dish, in a separate bowl, or just in the tank? Thanks!! 
