Is your leo currently in this 40 x 12 x 12 tank? If so, does it appear to be stressed out? My first thought is that yes, it would be awfully big for such a young leo, but, all leos are different, it may be fine. You could also just put a partition in the tank and make the area smaller for now 
I agree I would worry about it's ability to hunt crickets, but you could just cut off the crickets back legs so they can't escape as easily/quickly. And if you feed mealies or any other worm type feeders, those can be kept in a bowl.
I keep all my adults in 15 or 20 gal long tanks, I've only used a 10 gal for hatchlings. I just feel 10 gal is too small for an adult.
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