A rod and reel balances around your hand, so if you have an ultra light reel it cannot hope to balance the rod. Reels should be able to hold both your fly line and backing so that when recovered the fly line comes right up to the lip without snagging. A slightly oversized reel also lets you carry more backing. I have seen a salmon disappear with 200 yards of backing before breaking the 20lb test nylon tippet. Weight of the reel is immaterial since it sits between your hands and does less to balance the rig.
Many aficionados like to fish with old Hardy reels.
Not only do you have the pleasure of using an antique, but they are well engineered and are virtually indestructible. Look for the same qualities in a modern salmon reel. Rules for loading a salmon reel are the same as for trout - fill it to the brim, but not so much that it jams. You can get lot of flexibility from a single trout rod reel. Chose your rod and then your reel then buy at least 3 spare spools and lines to fit that reel before you buy another reel.