So, a little backstory might be needed.

I've had goldfish all my life but I've only recently been interested in the entire hobby - not saying that I haven't cared for my fish before, I've just really gotten into it now... like, a lot! I want to provide the absolute best care for my fish so I've come to fishy forums to ask some questions! 

I got our first fantail a couple of years ago from a neighbour and she was all alone and quite timid. Since then, she's received new tank mates and really perked up with her personality. She's currently housed with 2 other (smaller) fantails and, as of recently, a beautiful ranchu. Here's where the first problem arises: I also have peppered cory in that tank. (5 peppered in total)

Now, at the time of buying the cory, I was told that they'd be fine with goldfish (so long as they can't fit in their mouths) but, after some research, I'm wondering if they'd do better in a tropical tank? I have a heater in that tank and the temperature varies from around 20C - 23C. I also have a single albino bronze cory with them and this is where I'm completely lost. I remember reading somewhere that bronze and peppered are fine together but after some recent research I read that they like to split off into their own groups. Should I get another bronze for her and should she stay in the goldie tank? 

All the fish get along just fine with each other but I want to make sure that they're all completely happy and healthy so I need some feedback and help.

I also, very recently, found out that the ranchu is male and we now have a lot of little fry swimming around. I mistook them for cory eggs (as they have laid a lot recently) so they are in a tank (24C-26C) with the cory fry. I've been reading up all I can on goldfish fry since I've had no experience with that (though I've had experience with cory fry) but any tips will be extremely helpful to make sure as many as possible make it to adulthood. 

The goldfish fry are about a week old now so literally any information is great such as what kinds of food they like, when can I move them to a bigger tank, ideal temperature ranges, and culling. This is something I really don't want to do but I'd still like information about culling and when it should come into play. 

(Plus, the fry are currently on a mix of hikari first bites, liquifry no. 1+3 and the occasional bloodworm and frozen daphnia and brine shrimp. It's a big range as the cory range a lot in age, from 1 month to 1 week)

So yeah, any tips for the newbie fish keeper would be very much appreciated! 

Guidelines are recommendations by experienced goldfish keepers for those of lesser (or no) experience. They are not the only way of keeping goldfish, or even the best, but are our compromise between ideal conditions for goldfish and the space, time, and money limitations of the goldfish hobbyist. Koko’s guidelines are continually reviewed and updated as needed by the moderating team as we gain new information from research and experience.
Tank

Your tank should be large enough to provide at least 20 gallons (76 liters) for each goldfish. This is true for both long-bodied and fancy goldfish. While baby goldfish can do well in a smaller tank short term, they grow very fast, so we recommend starting them in their grown-up tank.  If you provide less than 20 gallons per adult fish, you should increase the amount of water changed to maintain water quality.

The ideal tank for goldfish is shallow with a large surface area. Tall tanks should be avoided if possible.  If you have a shallow tank or a tall one, use the surface area to determine stocking level -- 2 square feet of surface area per goldfish.  We recommend the 40B (40 gallon breeder) for two goldfish since it meets both criteria -- 20 gallons per fish and 2.25 square feet per fish.
 

Filtration

The most common filters for goldfish tanks are HOBs (hang on the back) or canisters. We recommend a HOB filter be rated by the manufacturer as turning over at least 10 times the tank volume per hour. Thus a HOB filter for a 20 gallon (76L) tank should turn over at least 200 gph (760 lph).  If one has multiple HOBs the turnover rates should add up to at least 10x the tank volume per hour.

Canisters have more filter volume, and we recommend they turn over at least 5-7 times the tank volume per hour.

We do not recommend internal filters for goldfish.