I disagree with you, they use goldfish as prizes to attract people and help them dish out a little more cash. If people didn't pay to play the game and the goldies weren't making them a profit then they'd stop using live animals as prizes. It's like when people gripe about bettas in cups, nothing will ever change because people keep buying them in the cups.
A 55gal is definitely a great start and the goldies will be much happier, you still wanna keep the water chages very regular and when they start growing the water changes will need to be more frequent. Manage the food you give them closely, goldfish do not have stomachs so they tend to eat as much as you give them. The more you give them the more the poop, the more they poop the more polluted the tank becomes. So I'm not saying starve them but feed in moderation.
The goldies could be comets, comets still get huge. So a filter rated for a 40gal is making the current too strong? I have two filters on my 55gal tank, one rated for 50gal and the other for a 100gals. Granted most of my goldies are a good size but I have one common goldie that is very small and he manages just fine. I'll soon be adding a power head to the tank to help push poo into the filters. I seriously think your fish will be fine with the filter, but you can still do the water bottle thing if you wish. Remember though that goldies are a very comical and playful fish. My large comet swims into the bubble steam and rides it to the surface of the water all day long. So it wouldn't surprise me if they were playing in the current.
Okay now for the hermit crabs, had them a while back and they were fun. Hermit crabs drink fresh water, not salt and not brackish. I don't even think fiddler crabs live in brackish...Anyways the hermit need to have a sponge to drink out of so that they don't drown. You can get one for cheap at a pet store, they're small and yellow. They also need a shallow water dish that the can crawl into, with water just deep enough to seep into their shells. Fresh water hermit crabs store water in the bottom of their shell to help them molt and also to drink if they don't have water nearby(something that happens in the wild). Hermit crabs need to misted once a day, they're a tropical animal so things need to be a little humid for them. You also need to feed your hermit crab a commercial crab food which you will also be able to find at your local pet store. Last but not least hermit crabs are social animals and they need a crab friend. They often die from stress when they are kept alone. Oh and if you can find a reptile heat bad you need to but it on the crabs enclosure, because they're a tropical animals they need warmer temperatures. Hope this helps.
