I'm going to weigh in here and say that according to all the serious fanciers of fancy goldfish the only fish that should be kept with a fancy is another fancy. This is my opinion, too.
This is further refined by tail and eye type. Single tails should not be kept with double tails, and normal eyed goldfish should not be kept with specialty eyed goldfish. The possible exception to this is the Dragon Eye or telescope eye; if the eyes are not too prominent, then the typical telescope - such as the so-called black moor or the chocolate telescope - may be kept successfully with fantails or other double-tail types with normal eyes.
The many varieties of Chinese goldfish have been developed with care over the centuries to reflect various aspects of Chinese culture. We in the west do not fully understand these principles, and to many of us fancy goldfish look ridiculous or, even worse, deformed.
This is not the case. The problem is that many of us here in the west do not get to see grand champion fancy goldfish or get to visit superior breeding facilities and study their techniques. We only know the selection at the local LFS. Even on-line offerings do not reflect the rich variety of goldfish breeds that exist; many Chinese breeder families have line-bred varieties which are never seen outside the family. Occasionally, an outstanding specimen may be brought to a national show.
In the book
Fancy Goldfish, by Dr. Erik L. Johnson and Richard E. Hess, there is a section by brothers Jackie Chan and Louis Chan of the Tung Hoi Aquarium in Hong Kong. If you know fancy goldfish at all, these names will be known to you. In the chapter assigned to them, the brothers describe the Chinese concept of beauty and how it is evaluated in goldfish by the principles of quality, size, and rarity. In these terms the much more elusive quality of charm -
shenyun - is discussed. All I can say is, I have never read a better written piece on the subject of goldfish. It explained everything.
Once you realize why each special variety of fancy goldfish was developed and how to judge an excellent specimen of each, you will understand why a large red oranda gives the impression of confidence and composure, similar to that of a medieval knight, and why the small-finned, stately lionhead gives the impression of an ancient Chinese lion dog, guarding the gates. I know I did. It was a beautifully written piece, lyrical and heartfelt. For the first time in more than ten years of reading everything I could about fancy goldfish, I finally understood what they are getting at with these various forms.
Poorly bred specimens will not give a good impression; they may be unable to swim upright and lack balance and purity of form. A well-bred fancy goldfish will swim upright and balanced in the water, and will have all of its attributes in balance and proportion. They live for many years in good health, despite that some of their internal organs are slightly rearranged. Well-bred excellent specimens will convey the particular aspect of their charm whether it's liveliness, stateliness, flirtatiousness, or bravery - to name but a few.
Goldfish are magnificent creatures in their own right and should be kept by themselves, with themselves. I suppose if andy's other forum poster was determined to keep fancy goldfish with a pleco, the domestic ordinary BN might survive it. Years ago, when such stores were common, I used to frequent a small appliance repair shop with a largish aquarium in the reception area. In it were two fancy goldfish of poor breeding and one large
Pterygoplichthys. All three fish were named "Chuck," btw! There was lots of filtration and apparently they had been living like that for years, but no fish was getting what he really needed and no fish in that tank looked well. All had shredded fins, but the owner seemed to think this was normal. The pleco had that sort of slightly grayish look to him, which implied to me that he had some kind of low-grade on-going skin disease. I won't even go into how the goldfish looked - let's just say pale, awkward, and listless. They completely lacked
shenyun.
I'm sure the guy thought he had a great set-up going there because all three fish were alive and had been for some time. Is that success to us? It's not to me.
I have three tanks devoted to catfish and each one is set up with the needs of the specimen fish in mind. I won't use the word "biotope" because I don't really believe there is such a thing in the hobby, but I will say successful "theme tank." I wouldn't dream of mixing fish with differing needs or compatibilities and I think we should discourage andy's forum friend from doing so. If you love catfish, set up a catfish tank with plants, wood, and tetras sailing above. Why throw goldfish into the mix?
ps: I used to keep fancy goldfish years ago. As we speak, I'm in the process of setting up a tank for them, again.
But please remember that I didn't tell anyone what to do. We were asked to give our opinions and I did. BeavisMom came to her own conclusion, even before she read this thread. My discussion of the development of the fancy varieties was really in response to Mats use of the term "deformed," not to push one culture over another! "Les chiens aboient, la caravane passe" or something like that ... 
