You can just feel the scion wood and flex it and tell if its the right hardness to use. It should also have buds forming in the crotch of the leaves. Im sure Ive posted some pics of good avocado budwood in the for sale section in the past. You can take cuttings any time of year as long as the hardness (semi hard) and buds are there it will work. It will work even with no buds but its better if buds are starting. They may be flower or leaf buds. Doesnt really matter. But the best time to take it is in December-March for my area. I think thats also the time they do it in south florida so should also apply to HI. But that is just my opinion, I see other people saying other times of year. But if you go read up on some technical papers by the old school avocado growers they are saying also best time is early in the year. You can graft later into the year if the weather is conducive. Its too hot and dry at my location past March to have good success grafting. Also once the warm weather comes, the trees flush and the new growth is soft and needs to harden off again to be used for scion wood. Thats one of the reasons to use wood from late winter, its hardened off already and about to pop for spring. And if you are in a proper avocado growing area with no frost, the grafts can easily handle the cool winter months and have ample time to heal before growing out.
As far as how much you can remove off the tree, thats up to you. You will figure that out pretty quick that even on a big tree you won't want to take a ton of wood because it sets the tree back and exposes the wood etc. Some varieties take that kind of pruning better than others. Nabal for instance always has the best scion wood and can be pruned hard. Other varieties not so much. You will figure that out by doing it. Same goes for getting the timing down and when your trees have the best budwood and when to graft.
That said, most sellers dont care and are just out to make a buck and will cut it whenever. Or the buyers dont care its the wrong time of year and want to graft on a whim etc. People ask me for avocado scions all the time and I usually prefer to just collect it in January or February because I know thats when my trees have the best wood. And I know they will have best results grafting then. Of course some people are able to graft in summer if they live in a mild environment or are skilled but generally speaking late winter, early spring is best.
Good luck with the trees. Growing avocados living in Kauai sounds pretty great. Maybe grow some mangos and coffee, get a couple pigs to cook, you be in paradise.
