Think you need to make a decision. There is a pretty big difference between 30-40 inch steelhead, and the occasional 24" bow river rainbow. A 7 or 8 weight would do you a lot better for steelhead then trout, and you can always just deal with having a big rod. If you go smaller, like a 5 or 6 weight spey to have more fun for tout, you're going to be undergunned in a big way for steelhead (to the point you probably shouldn't use it, unless you're doing dries on the Morice or half-salts down in the states). If you were just trout-spey fishing, i'd say look at a 5 weight as you can easily turn over big streamers with that as it's rare that you're using big sink tips on the Bow. If you're looking at a do-it-all, a stiff 7 like a TCX would be a better option. If you're leaning more steelhead, then an 8 would probably be a better choice.
Also, 'big' streamers doesn't mean much. Are you talking about 4" intruder style, 6" sex dungeons, or 10" monster bugs. They're all completely different in how they cast and what will turn them over.
I remember when the spey fad hit here in a big way and guys were using 8-9 weights so they could turn over massive bull trout flies, until they realized that a 13' spey rod was essentially useless on the oldman/sheep/highwood. If you're just wanting to chuck seriously big bugs, you're not necessarily looking at the right tool for the job..