Isotenny, having started out with such a mismatched tank as yours, I definitely know where you are coming from. But as Kashta said, those of us with more experience try to help out the newer fishkeepers and prevent problems before they happen. Let me be clear: we're not trying to be critical or put you down. It's catastrophe management, really.
Let me give you a quick example of why some of your species are incompatible. Goldfish require at least 15 gallons of space per fish, so your tank is currently holding exactly twice the number of goldfish it can support. Since they produce so much waste, goldfish need a filter rated for twice the tank capacity. Therefore, if your filter is built to filter four times as much water as is in the tank, it is just barely keeping up with the number of goldfish you have in there, let alone your other fish.
Bettas are terrible tankmates for goldfish, because they do best in temps of 70-80 degrees, require very clean water to prevent finrot, and need a high-protein diet that is incompatible with a good goldfish diet. While goldfish are neutral-pH species, bettas do best in soft water. Your cories, loaches, plecos and catfish are tropical species, while dojos are coolwater species. When the goldfish get large enough, they will attempt to eat any fish that will fit in their mouths, endangering the danios and kuhlis. If the fish is a little too big, it can get stuck in the goldfish's throat, killing the goldfish.
Not to mention that plecos and kuhlis are both very sensitive species, being scaleless and should be removed from the tank before any medication is added and treated separately at a much lower dose. Coppersafe is not an effective treatment for gill flukes. Flukes of any type need to be treated with a parasite-specific medication like praziquantel or formalin. You need a treatment that will kill not only the adult fluke, but the juvenile stages as well.
The idea with fishkeeping is to prevent any tragedies and provide the best environment possible for the fish you are keeping. Much like children (you will know about that, with your child safety course), fish are vulnerable to the environment they live in. As caretakers, we have to be responsible for their wellbeing and provide an appropriate environment. It prevents stress, illness, and tragedies, but also gives us the opportunity to witness interesting and wonderful behaviors. Just like with kids, prevention is the key to health. No parent plops a child down in a park and thinks, everything must be fine because he looks great right now. The broken glass in the sandbox, the peeling paint on the swingset, the barking dog across the street - you have to consider the future for your fish and do what you can to keep them healthy and happy.
Best of luck to you and your fish.