You have a rather extensive list of questions, so I'll do my best to answer them in an orderly manner.
To begin, a 29 gallon aquarium is an excellent candidate for a paludarium, due to its extra height (which thereby permits ample space for both aquatic and terrestrial habitats.) It is, however, not terribly deep, so any background needs to be relatively thin to permit you as much space as possible.
In terms of occupants, if the golldfish is the one fish that you are set on displaying, you should really keep in mind its specific needs- like its preference for sub-tropical water temperatures. With the exception of the koi (which I also agree have no
place in any
aquarium under 100 gallons,) all of the fish that you listed are tropical species, and would suffer in the colder climate necessary to keep your C. auratus
at its best. If you're going to house goldfish, stick
to goldfish and other specifically cold water species.
Now, in terms of the two amphibians that you listed, the African dwarf frog is an unacceptable tankmate for the same reasons listed above (it may even look like an appetizing snack to a large, hungry goldfish.) Many species of newts, however, flourish in temperatures from 62-72 degrees Farenheit, and could, potentially, live with your fish; however, again, a swishing newt tail may seem an appealing snack to an inquisitive goldfish. Due to the potent tetrodotoxins housed in the glands in nearly every newt specie, I fear your unfortunate fish would have a quick end, should it ever take a bite of its four-legged tankmate.
Goldfish also make difficult tank mates due to their rather prolific penchant for producing ammonia. While they are quite hearty and therefore able to withstand the result of their overly-active metabolisms, many potential inhabitants could be quickly overwhelmed by chemical accumulations in aquaria with lesser volumes and levels of filtration. This, therefore, brings up another difficult question: if you are really intent upon featuring your C. auratus
in this tank, why decrease the volume of water if it will only disadvantage the tank's "star?" Perhaps if the majority of the tank is aquatic, with only 9 or so gallons utilized as the terrestrial section of the tank, it would have little effect, but part appreciating a goldfish is providing it with enough space to really see it swim.
Anyway, it's rediculuously late (early?,) and I'll have to answer the rest of your questions tomorrow (or is that later today...?)
Good luck, and keep us posted.