First off, I suggest a LOT of LR, at least the whole length of the back, as lions are deffinitely reef dwellers and will benefit highly from the sense of well being this gives them.
As long as you're using a sump and a good skimmer, you should be OK with a relatively shallow sustrate.
We've had the best luck by using VHO lighting and lots of macroalgae encouraged to grow on the LR, as it helps with the nitrates that come from well fed predatory feeders (nitrogen=fertilizer)
As to you tank size, we'd suggest dwarf lions, fuzzy dwarfs, antennatas and or FuManchu lions (most get along very well, with the exception of the occassional beligerant male, particularly in fuzzy dwarfs)
If you're going with smaller lions, rather than a grouper, might I suggest instead either medium/large hawkfish or hamlets(these might not be available in australia, being Caribbean, but the blue offsets the lions red very nicely)
Some of the smaller Aussie sea bream (blues and yellows) would also work well.....they aren't commonly available in this area, but we've enjoyed any we've ever had and are continually on the lookout for them.
Another possibility with your long, low tank would be leaf-fish, sea goblins and other smaller scorpions, as they are bottom dwellers and mix very well with no aggression, pls have the same food requirements.
