I fish the Credit as do a lot of other guys here. An 11' to 12' rod is enough for swinging a fly on that river. Given some of the deep holes, I use a range of shooting heads/Skagit lines from floating to full sinking, depending on flow rates. I prefer the 12' rod for the extra reach and ease of covering the water, especially when using full sinkers. I do use an intermediate Skagit setup for the 11 footer and that works, but it requires a soft touch to lift well. The 12' just makes things that much easier.
As a general rule, I fish the 11' rod with a Skagit setup when the flow rates are low, switching over to the 12' rod and sinking Scandi setups when flow rates climb.
I've fished it with rods as long as 13' 6" but that ended up as overkill. No need for anything more than a Scandi or Skagit setup, except for winter days when guide icing is an issue. On those days a 55' line will do the job.
If you want to also fish for chinook with the swung fly, consider a short 9/10 rod. A good sized steelhead will still put a decent bend in it, but a chinook won't break it.
I strongly suggest some casting lessons before investing too heavily in tackle. It'll save you money in the long run.
PM me if you'd like some more info or clarification on some of the terms I've used in this post.