Since my English isn't very good, I wonder if I got your question or not.
First of all, 島 is noun, it isn't an adjective, and it's read SHIMA or TOU. Besides, in this case, they are a part of the names of islands, it's been called in that way for a long long time, so I don't know why some are read SHIMA and the others TOU. The same happens for 町, some are called MACHI and others CHOU.
However, as one word, 島 is SHIMA, 町 is MACHI. (Ex. わたしはしまにしゅんでいます。わたしはまちにすんでいます。)
I hope you get it even though a bit, and I really hope other people explain about this question better than me.