I've been using my 7 wt switch rod and found that I can retrieve flies no problem. I think it's better suited than using a full length spey rod to cast and retrieve. Plus, I get the benefit of higher efficiency when drifting the fly (no false casts). However, for frog water or at the beach (salt), I'd go with a single hander for cast and retrieve for sure.
For chums and springs, I've probably hooked more fish on the drift. I have hooked them on the retrieve, but I find that coho and pinks seem to chase the fly more than the springs and chum.
Forget about using your 6 wt for the larger salmon. You will be under-gunned which is bad news for both you and the fish. An 8 weight rod is a good all around rod for salmon/steelhead. Even an 8 weight can be slightly under-gunned with a spring, but at least you'll be able to put some amount of pressure on the fish when you need it. I also prefer a fast action rod for salmon. I find fast action rods are better suited for tossing heavier sink tips. Plus, with today's graphite technology you can get a fast action rod with good sensitivity. Check with the rod manufacturer or fly shop about the "action" of the rod.