A lot of your questions depend on where you live...
I read Big Island in your other thread, but even then it would help to know what side of the island you're on.
Don't look to be shooting Tuna off the bat. I've been blue-water diving a bunch of times there & never even saw one!
Lots of Ono (wahoo) and Mahimahi(dorado or dolphin fish), but no Tunas. Great ambition to look forward too, but just don't want your expectations to be too unreasonable.
Euro guns work fine inshore. I've taken many good fish with them. As far as the pole spear - it's very commonly used in Hawaii, but get a 3-prong (or paralyzer tip) instead.
As far as fish edibility... that'll take a while. I'll get back to you on that one. Most of the fish are good eating.
Give us a little more info on where you are & we should be able to help you out a little bit more.
Oh yeah, check out
Hawaii Skin Diver for more info and pictures on spearfishing in Hawaii!
