Hmmm those are tough questions. First off hey..we all started with gold fish..i still keep them...i have almost the same as you...a black moore...the one with the bulging eyes..lol and a gold calico fantail..these two are proven breeders and i will never give them up..they are also about ten years old. Im not much of an expert on crabs...ill tell you what i do know..yes there is some freshwater ones..infact there is several types...getting these is a matter in itself. Any type of freshwater crap is hard to get..there just isnt the demand. Most that are available, actually need a little salt in their water..which more or less makes them a brackish water crab...ie..fiddler crabs, and they need to come out of the water. As for suitable with goldfish, im sorry..but i just dont know of any...any that will either A-be attached by the goldfish for being too small...or B-eat the goldfish if they can catch them...sorry
...but do some research..you may be able to prove me wrong..and if you do, please let me know as i would be interested in going in with you if you have to import them