I fish them with a 6-wt in small or low water. I use 10-lb. Maxima for tippet and sometimes bend out hooks pulling on them, so I think that's strong enough. For the way I fish them (lots of casting and stripping), a 6-wt. Is a good balance of light in the hand yet strong enough to tame the typical coho. As the water gets higher, I go to the 8-wt. (And the gear rods LOL).
I have landed small chinook on the same rod, but it's been very "iffy," and it was about a 14-lb. King that broke my first 6-wt., so I no longer target kings on anything less than an 8-wt.
Of course, not all 6-wt.s are equal. The one I'm fishing is an Echo EPR, which is designed for shooting long casts, and it has a very stout backbone for a 6-wt., so it handles coho much better than others I have used. A softer rod would make casting the line and heavier flies harder, but it would probably be fine for wrangling the fish once hooked.
Editing to add: An Airflo Streamer Max line with a full sinking polyleader (furled leader in my case) of about 4 feet, then 3 feet or so of tippet has been an ideal setup for me. The 6-wt. Line (don't know the grain weight offhand) is really easy to shoot up to about 60 feet (and can be thrown much farther if needed), and the head sinks very well, so your fly stays in the zone while you strip it back. (It's not great for swinging shallow-medium flats, probably because the sinking head is so long, so keep in mind where and how you're fishing.) I hooked about a dozen tide fresh fish this way the other day in the same spot, so I think it might be a winner.