Sinker, as far as my thoughts, well, they might not be the best advice to follow, as my experiences nymphing with a spey rod have not been terribly numerous. What I did find was that 8/9 lines made it less tiring and easy to spey cast the "dude bobber" rigs commonly used. But I never really tried to fine tune a lighter set up to make it work well. By cutting into the front taper of a short belly line, I would think it would be easy to make a 7 wt do the trick. I just never experimented enough to get a set up I really liked, as the Spey rod I was using was a shop demo, and I prefered to nymph with my singlehander. This was a few years ago when I was living in Michigan. There are a few Great Lakes guys who commonly use this site (MYJP, Voodo fly, Carl, Peter SC, numerous others) who could probably consult you with more first hand experience.
As far as the 8/9 vs 7/8 for fighting the occasional salmon...well, some of those brutes are unlandable with a 12 wt, some don't do a whole lot and are easy enough on a lighter rod. In rivers with a lot of obstructions for fish to toast you on (the PM immediately comes to mind) a heavier rod will have a little more backbone to lean on the fishys. But, a lot depends on what you fish for tippet. If you are one of those 2 lb guys, a 12 wt isn't going to be forgiving enough to play a fish, even though it may have lots of power. Also, a 7 wt with a stiff butt may be more powerful for playing fish than a 9 wt with a soft butt. Line wt isn't always the biggest factor. What all this adds up to is, TRY the rods. They can tell you more in a day than all of the info you can find on this site.
Also, you will surely end up with more than one spey rod if you get the bug, so maybe go into your shopping with this in mind. Man, I am rambling. Sorry, I get carried away. I hope this makes a little sense.