FYI: eagle is pretty cool, you start with a schematic then layout the components on a board.
re: fuse, you can certainly add it to where you tap into the power supply, it wouldn't hurt if you are using smaller wire than the existing circuit protection will provide for, but it doesn't really belong on the board itself (most devices do not have such a fuse). The 100k resistors will prevent calamity on the signal lines.