Hi
You have picked the wrong islands for wrecks! Well there are some of course but not worth coming here to dive. I'm not sure which wrecks Hodge Podge dived, but resort based wrecks you are looking at 120ft max.
Plenty of big walls though. Ask Richard Pile and his iMax team.
Solomons or Vanuatu for wrecks.
As for gear and gas. The only bulk supplier is BOC in Suva. They have plenty of O2 but no He. Well they did have 3 J's a year or so back that were gathering a lot of dust, but they had vanished when I last snooped around the depot. I can check for you if you like and find out if they can get some in. Be prepared for a nasty surprise when they tell you how much they want for diving grade helium though.
You won't find anyone really doing techie stuff here. A couple of resorts and both main live aboard boats have O2 and boosters as well as membrane Nitrox systems up to 40%. For fills over 40% you are going to need to do partial pressure fills so bring your own decanting kit, filters and adaptors.
Tanks. No resorts are diving doubles here, but nothing to stop you bringing bands and twining up some as independent bottles. If you give me some notice, I can probably make you up some manifolded sets if my box of parts is still under the bed

Almost all the tanks here are 12 litre, aluminium at 207 or 240 bar. (Thats 87 and 100cuft in American I think).
There are a few smaller tanks knocking around the resorts (60cuft or thereabouts but not very many) Give me some time and I can round some up for you - probably.
None of them will have any rigging so bring what you need.
So realistically you can forget trimix but it will be possible, if you organise it right and go to the right resort, to get rebreather fills and nitrox.
For your free divers, there is far more scope. We specialise in laid back boat trips in Fiji. It will be easy to arrange with almost any resort especially in the afternoons when the diving has usually finished.
Anyway, if you tell me a bit more about your planned trip, numbers of people, rough budgets, preferences etc, then I will be happy to suggest some locations.
Cheers
Greg