I think the goldfish and pleco thing is a good topic. I guess they are no more messy than anything else their size. I think the "myth" if we can call it that is from considering the size of the fish in relation to the tank. So yes, a common pleco in a 10g is eventually going to be very messy, even if it's only 2-3", because it will get big enough to cause problems with excessive waste. But then we get into generalizations too. Saying pleco or goldfish in the way Bob is talking about usually means common pleco and comet goldfish, when in reality pleco is about as accurate a name as cichlid.
And since this is a rant thread, we are doing a series on Jonah at church. As a prop, the pastor from last Sunday brought in his daughters guppy bowl. Yup, you read right, a single male guppy in a little 1/2g bowl complete with day glow gravel, plastic plant, and treasure chest. My efforts to tell him why it was wrong, and even my offer to buy him a 10g setup went unheard.