CardBoardBoxProcessor, I have heard that bristlenoses don't eat diatoms (brown flat algae), or BGA (blue green algae, actually cyanobacteria). As far as off plants leaves and such, I've seen pictures of some major damage bristlenoses have done to plants, eating not just the algae but rasping so hard they eat big holes in the leaves.
Uh, I don't even know what to say about the goldfish thing. The plec needs warmer water to mention one of many reasons why I would never put one in the same tank as a goldfish.
xJake, I don't think BGA is harmful to fish if consumed. At least, not all fish. I have heard of a vanamenia species that may eat it. But I've also heard it may contain toxins. Eh, now that I think of it, everything I know about it just hearsay. I have a little in one tank and am going to have to take care of it but as far as I know covering the tank for three days (no peeking! a complete blackout, according to those in the planted section here on TFF) will kill it, as well as antibiotics combined with reduced lighting.
I'm not a plec expert, but if you ask me, plecos shouldn't really be looked at as having a cleaning responsiblity. They're pets with their own needs, and may eat some algae if it looks good to them. My one plec does eat whatever he wants in the tank, but as far as how clean the tank looks--thats my job.