you would only have to make adjustments in the few parts like front seat rail, front seat slat. to allow for thinner legs.but why decrease the thickness, 5/8 th would be fine.what started me making these smaller chairs was that I went all the way once to staten island to someone who claimed to have a torn down new pT deck, in great shape, he just didnt want it anymore after he bought the house, and he offered it to anyone to cart away.
It was all small pieces, they cut it up so there was nothing over 2 feet. I took some of it, figured its good for something, and it was, small kids chairs. nothing over 24 inches.
I like to buy 5/4 cedar decking when lowes has it on sale, really cheap, and I love white cedar for chairs.