For salmon fishing on the bottom sections of the RiversTaw and Torridge and for most of the Tamar up to and including Endsleigh, a 12 – 14ft double hander is ok. A single handed rod of 10ft rated 7 or 8 is ideal for smaller rivers and summer levels on the aforementioned lower reaches.
For sea-trout in all but the smallest streams a 9 – 10ft rod rated 6 or 7 is ideal. An 8 weight is ok for night fishing the larger rivers.
For brown trout an 8 or 9 ft rod rated 3 – 5 is great. A 6 -7ft rod is good for Dartmoor and Exmoor streams and the modern practice of using 10t rods rated for lines as light as 3 as part of a nymphing outfit can be exercised on rivers like the Exe, Taw and Tamar.
If one rod to do all things were needed, it would probably be a 9ft rated 6. We would advise however that two rods are the bare minmum. A 9 footer rated 5 for trout and daylight / small river sea-trout and a 10 footer rated 7 or 8 for salmon and night fishing. The latter is better than the 6 as often a sunk line technique is the best method at night and the rod needs a bit of backbone.