Fuse the neg wire. You should fuse each load. Getting a bigger fuse to serve two devises is not a good idea and almost pointless. As far as wire sizes go bigger is better. You can mix match sizes just don't use wire that is smaller than the factory wire on the devise you are running to. Also I would put a fuse between your block terminal and battery. It could be pretty big say 15a just in case. If something conductive were to fall on or lean against the buss it could get ugly. Hope this helps.
One other thing. You can get a time delay fuse for your pump if you really want to do it right.
