Well answering both questions...Yes they can fly but VERY few will. As far as escaping through air holes...they are a hard shelled beetle, SO if the holes are big enough then yes they will but you should not have air holes big enough for them to escape.

Mealworms have approximately 5 sheds before they will pupate and then 2 weeks or so to become their adult beetle form. The only way to slow this process down is to refrigerate them. Depending on what you are feeding you can also consider superworms. These will not pupate while in a colony so you have "worm" for a much longer period. I personally breed my own supers and buy mealies only when I don't have appropriate size for babies or sometimes geckos will refuse supers due to the defensive fluid they secrete sometimes. This fluid is similar to the stink bug fluid when a beetle, but has no smell as a worm.