Although I did this professionally for nearly a decade, I don't really have much advice on how to run wires as every situation is different. Just be prepared to drill holes through things you can't see, and make sure you have a good fish tape to help pull wires. Try to stay in front of any insulation you encounter. Also, if you are pulling more than one wire from point A to point B and you might want something different (or to replace a cable later if one goes bad), run some heavy duty string or even an extra 16-18 ga wire along with the other wires. That way you can just attach your fish tape to the extra wire or string later and pull it through, attach your new cable and just pull it right back without all the headache of fishing through a hole you can't see.
Those wall plates can be painted to match, which will help blend them in.
The "open back" rings are great for anything low voltage.
The only other thing I might mention would be to look into Baluns for video or audio over long distances. They aren't cheap, but if you are running more than about 50 feet, they are the best way to go, and you just use basic cat5e cable. If you are pushing that length or just over with HDMI, make sure you get redmere cables.
If you can't find a filler or dowel that looks right in the holes in the floor, you can get plug cutters for a drill and you can find a piece of whatever kind of wood the floor is made from and cut your own plugs and just glue them in and sand flush and stain.. if you are careful you can minimize the sanding and blend it in so well the patch will disappear completely. The dowel thing works but you end up with end grain and not face grain, so once you stain it, it will be darker.