The substrate I would use over the LECA is sphagnum moss. It should keep any other substrate placed upon it from sinking below the egg crate. Also some plants will grow fine in sphagnum. I have two bromeliads in my dart tank, their roots hang down to the reservoir. Some other plants have popped have popped up in there too. I have no idea what they are or where they came from. The substrate should be at least two inches, but it can be thicker, thats including the LECA.
I think in the size tank you mentioned, 6 - 8 clowns will be fine. They are very small frogs. You will most likely be able to get some next spring. When you do get them, don't throw them in immediately, quarantine them elsewhere for a minimum of 30 days, 60 would be better. You want to make sure they are healthy before they meet there new home, that way if they are sick, they won't infect the tank for the next occupant.
Not sure if the clowns will eat the springtails, as they are very tiny. They might, I just don't know for sure. I think the fish will though.