Hi there.
By South Coast track, I assume you mean Cockle Creek to Melaleuca, or vice versa.
Arranging a food drop half way could be difficult and is not normally done. Certainly it cannot be dropped by air, so all you could do is carry it in and stash it in advance, or get in by boat somehow to drop it off.
Normally food drops are left at Melaleuca for people walking through Melaleuca to other destinations (combining the South Coast track with the Port Davey track to Scotts Peak for example).
Last week I saw about a dozen leeches in the 10 days we were there, but none got us -
DEET is the answer.
Flies weren't much of a problem for us, but could be at that time of year. Again, DEET for the person, but the biggest threat to me for flies is when they land on your food or utensils having already visited unsavoury pits and the like - risk of gastro. Vigilance when preparing food is about all you can do.
I saw a couple of ticks too so watch out for them.
It's quite a demanding walk, with a few particularly tough days - crossing Red Point hills, over the Ironbounds, and also the South Cape Range which was particularly muddy.
What age group are the "youngish young people"?