Just GO for goodness sake! There are SO many salmon rivers in Newfoundland, and little-no private water. There's lots of relatively-inaccessible water, but it's not private if you can get to the riverbank.
This being said, there are great salmon rivers, easily accessed by road, all over the province. You can get into whatever kind of water that you love. Big water (e.g. Humber, Gander, Exploits), smallish water (e.g. East, Torrent), brawling streams, still pools ... EVERYTHING!
Guides are mandatory, but usually not too expensive. All will know their local water. Some will be excellent by any standard, others - while being fine guides - might challenge your expectations of the guide/sport relationship. If you go with an Outfitter I'd expect your experience would be first rate - with all of the usual bells & whistles.
The most popular spots (e.g. Humber - Big Falls) will be crowded during the summer runs, and the local anglers tend to not rotate through the pools. Still, you can get a good spot at most of the busy places. If you want secluded fishing, just ask your guide. You'll be able to drive a few miles and find your own pools - or river - to fish for the day.
I've fished the Codroy, Humber, River of Ponds, Torrent and Big East, and I loved them all! Other anglers could list up to 50 more rivers as their favorites, and all of their choices would be just as good as mine. Water levels will profoundly affect the smaller rivers. Quite a few of them do not fish well in low water, and the Fisheries can be closed by the Government in season. This is not a problem on the bigger rivers, but you have to be flexible if you hope to fish some of the smaller gems.
I went in early July for several years in the late '90s. I caught fresh-run fish - mostly grilse. This may be changing now, as the MSW fish aren't being netted out of the ocean to the extent that was going on in the '90s. Still, most NF rivers host larger grilse runs than MSW fish. This being said, the late-season fishery on the lower Humber is all about catching the large fish than come in later in the season.
If you go in prime season, it would be hard to not get into fish. It can be slow, but no worse than any other salmon fishery in North America. Best of all, there is another great river up or down every road or shoreline. It's a superb place for a 'plan as you go' salmon trip. And, you will never forget the country! The natural beauty is beyond description and the rivers are spectacular.
Oh ya - don't plan to drive any distance at night! The moose & caribou are a very real hazard on the Rock.
Enjoy your trip!