Hi Darren,
Your #6/7 will be fine, I often use a stiff #6. Think you've gone about this the wrong way mate, I can't see anyone here sharing much knowledge with you after voicing your intention to knock a fish especially in a season when salmon have been so scarce. You buy your ticket and you have the right to take a fish should you so wish but I would maybe think carefully about what you take, small cock fish if anything and make sure it is fresh otherwise your efforts will be wasted. Salmon once in the river for a while taste pretty grim, not sure how much salmon fishing you have done in other places but if you're not sure about what makes a good "keeping" fish, contact the Biologist at the Don Trust and he'll help you out.
If it was me, I'd be taking and would rather see a well taken pic of a salmon being returned than a dead fish in the flesh.
Cheers
Ben