Tim,
I agree with you on not mixing species. Most home aquaria are too small to even consider mixing to begin with. Unfortunately even popular books and magazines (at least here in the US) often continue to advocate mixing. I was reading a childrens Q&A column in a popular aquarium magazine last week. One of the questions had to deal with mixing frogs and goldfish. In this letter the kid also mentioned having a 20 gal (I think) with various fish, newts , and turtles of all things. The magazine answered that the fish and frogs should be separated however nothing was said about the 20 gal tank. I found this to be irresponsible as saying nothing can be as bad as giving permission. Another author of a very popular series of books also advocates species mixing (different species of anoles, fish, geckos, frogs, newts, green snakes, etc). I feel that this case is particularly bad because the author otherwise gives excellent advice and I often recommend his books. With all of that being said I think that it would be fun to setup a 300 gal tank with either some paramesotritons or pachytritons and a school of white clouds.
Chip