A mate of mine regularly catches jew on fly, his largest going 17kg. but keep in mind that this comes from many years of targeting jew on bait and lures and having an intimate understanding of the fish and thier patterns. He wrote an article on this same subject a few years ago in saltwater fishing. It is very possible but it is a rather difficult goal you have set yourself.
As for soapies being found in the same places as mature fish. Juveniles have a very different dietry requirement to mature fish. so it is most likely that they will be feeding in different places and situations. learning these patterns is the real key to success.
i do not believe mulloway exclusivley feed in the bottom third of the water column. They are a predator, and when actively hunting can be found anywhere from in amongst tailor schools in mid water to smashing mullet and gar off the surface, or found patroling a hole in 40ft of water.. and the next bloke to pull a 30kg jew out of a 6ft deep beach gutter wont be the first. I dont think mulloway take residence in one particuar part of an estuary, but rather roam using the influence of the tides and weather conditions to exploit particular feeding opportunities, where ever they may present themeselves. I believe most fish that are targeted and caught fishing deep holes ,are not necissarily because the fish are actively hunting these areas but because they are moving through. Like putting a cheese burger out on the road to maccas, some hungry bugger is not going to be able to go past it without having a nibble.
Current and respite from that current, along with structure will hold bait, and at certain times predators will try to take advantages of these congrations of bait. So, observe, learn, understand and you will succeed.
Good luck