I agree with the advise posted Previously going with the 8wt. it should give you a little more flexabity especially if your useing a sinking line and or wieghted flies. I've caught fresh silvers on my 5 and 6 but my 7 casts the flies easier especially
the wieghted bunny flies.
I 've never fished the Kenai but have heard stories about the crowds. It May be a little lest croweded in August then earlier in the year. I was in central Alaska around the 17th last year and heard the Kenai got a great second run of reds and almost no one was there to fish it. As far as trout it's my experiance that when the Trout are keying on eggs thats about all they will look at pure protien. If there are no eggs standard trout flies will do along with ESL's and buggers. Do some searching on the internet there is alot of info out there.
Good luck those silvers are FUN!!! hope you hit them