am I crazy (goldfish with tropicals)

Well even though the tank is cycled I heard that goldfish secrete something that is bad for freshwater fish, not to mention that goldfish are very wasteful fish. But if you do enough water changes I don't really see why it would be a big problem.

Loaches yes, plecos more or less, ottos... No, but in this situation it'd work. I doubt there'd be any behavioral problems so I wouldn't even bother with the breeding net. And I don't think there is a secretion emitted by goldfish that would kill tropicals...

I thought goldfish were colder water fish. I could be wrong

I think putting him in a breeder net is going to do more harm than good really because he will be confined amongst a full tank. If the goldfish is small, I would say it's ok for the treatment and is much better than keeping him in an unfiltered tank but do take caution because goldfish are large bio-producers.

The issue with them secreting something harmful, I've never heard of that but I would guess that it would have something to do with the hormones they produce. They grow fairly quickly, producing a lot of hormones to the water, as well as adding a lot of waste to the tank which will entail a step up in the water changing of your filtered tank. Even though you say your tank is cycled, adding a goldfish will add a lot to the chemistry of the tank, possibly throwing the tank into a min-cycle. Just something to be careful for.

As far as the otto's and the goldfish, aside from them having different needs long term as far as temp, my guess again would be that they are not good tankmates with the goldfish because as the goldfish grows, so does it's appetite. If the goldie can fit it (smaller fish, debris, gravel, anything) in it's mouth, it will try. That said, I would imagine that a goldfish may try to snack on an otto if given the chance and the goldfish is big enough. I am not sure if ottos have the same spring action that cories have but if they do, once swallowed (or tried), their fins sprinbg open and become lodged in the fish's throat.... often causing the demise of both of them. On the other hand, ottos are pretty quick so could avoid this easily however, ottos may try to attach to the weak goldfish to suck on their slime coat, causing further infections. Short term, this may not be a problem though. I think what your bigger issue is that you need to get a filter for the goldies tank and get it cycled. You can do this by sharing the existing mulm from your established tank into the new filter.

the confined space may be stressful for him.... the higher the stress, the harder it is to fight off infections. Also, because they are high waste producers, it may be impossible to keep the breeder clean enough for him. Even though a breeder net/trap is in the main tank and can allow airflow, it's not ideal and the water flow isn't sufficient enough.