Good choices in rivers for that time of year if you are after salmon and perhaps the odd steelhead.
Both rivers have well developed trail networks for walk and wade fishing. And both river has trail maps available at local fishing and tackle shops, so you should be able to find your way around if you want to fish without a guide.
That being said, if you want to increase your chances, hire a guide for a day on each river, and you will be sure to catch plenty of fish and learn enough to fish on your own after that.
Unfortunately, since the Cowichan, Stamp and Campbell rivers are quite famous, you will likely be fishing next to people all day long unless you are willing to hike into remote places. Guides should be able to help you have a more private fishing experience as they know the river well, and have the resources (boats) to get you to fishing spots that are both productive and hard to reach.
Have fun planning your trip! If you base your fishing out of either Duncan (Cowichan river) or Port Alberni (Stamp River), ask your guide about fishing other rivers. There are plenty of more remote and "wild" places where you can find more fish and fewer people.