Yeah you can leave mealworms in 24/7 in a bowl (specially made 'mealworm bowls' or candle holders and similar work). I would make sure they are dusted and add a small bit of veg in the bowl for them to munch on. Keeps them well gut loaded but also wriggly so the gecko can see them better. Mealworms can bite but I find the geckos flick them once they touch their feet so don't get much of a chance.
It would be worth giving locusts and cockroaches a try, these don't bite the lizards so can be left in.
I wouldn't be too worried about leaving a few crickets in there over night, I would just add a small piece of veg in there for them to munch and remove any survivors in the morning. As fatties are active and hunt at night, leaving a small amount of crickets should be fine.