Depend on the size of the lungfish and the aggression of the lei. If the lei is way to aggressive then no, if not then go for it.. As for the substrates you could go for both but I prefer sand. Much easier to clean as you can spot the poop right away and vacuum it up.
Cool i prefer sand aswell, just didnt know if it was alrite for AUL,
i havnt seen my lei show any signs of agression to any of my tank mates yet so i think i can get my AUL when i save up enough thaat is haha