I live up the street from Fanno, and bike the trail all the time. First, I'm quite confident that coho spawn in Fanno. A good chunk of the coho over the Falls are heading to the Tualatin, and Fanno is one of the first big tribs upstream.
I believe most of the cutthroat population is up high, in the colder, clearer, faster water - like around Hillsdale. In Beaverton and Tigard, all the culverts and whatnot make it a big stagnant pool for much of it. I think there's a few pikeminnow, and I've seen kids catching... are those things called "dace"? And at the lower extreme/mouth, there's carp and squawfish o'plenty (accessed through Cook Park, Durham Park, or Tualatin Community Park). But the Tualatin is high right now (and probably will be for a couple of months -- sucker doesn't drop very quickly).
I'll occasionally tie the ultralight to the bike and go mess around near the mouth. Mostly pikeminnow.
If I was going to make a trip to check out Fanno, I'd probably go around Woodard Park (which is right down the street from me), which has some flow/broken surface, maybe down towards the library. Above Woodard (east of Scholls Ferry), I don't know if it's beavers, or some ill-advised habitat restoration, but there's pile of debris, which bring the flow to a near-standstill for quite a ways upstream (all through Greenway Park and beyond). You can't really get to the creek above Denny Rd for much of that stretch. I don't know of any access in the Hillsdale area, which is where I've been told stories of cutthroat (all small).
Regardless, I'd check the regs before you go.
I'd love to hear a report if you go.