Steve,
You can easlily combine a 7/9 wt reel with a 6/7 wt rod. It might be a little large, but that's just looks.
The line you need must fit your rod - 6 or 7 weight - no matter what you fish, trout, salmon or whatever. The line and rod must be the same weight.
And yes, you need backing. You need to fill the reel with something and also want some backing in case you hook something big. 100 yards or so will usually do, but your reel can probaly hold more due to its size.
The leader should reflect the thickness of your tippet and the tippet is decided by what flies you are going to use: small flies, thin tippet. Large flies, thick tippet.
Fishing loughs in Ireland may mean fishing several flies in a team, and for that you might find some inspiration in the article about the Diawl Bach.
Martin
