Are we talking Rainbows as in Cichlids? I don't know that Rainbowfish would stand a chance against hugely aggressive Cichlids. To that end, I am really not sure about the Giant Danio. I think that those fish might become either food or be forcibly ejected from the tank, if not killed because of lack of hardiness.
You also didn't say, but I am assuming that your Plec is the common one (Hypostomus sp.) He might cause some filtration problems for you in the not-too-distant future (see the catfish and bottom-feeder forums for details about him). If he is a bristlenose (Ancistrus spp.) or some other smaller Plec, I would worry about inability to hold his own.
You might want to be diligent in watching the paired fish, because they have the ability to make life miserable for their tankmates if they were to start breeding. Plenty of shelter is recommended.
Also, I don't think that there is a species of Cory which will stand up to the Cichlids. The largest ones are generally the most fragile (C. barbatus, C. semiaquilius, and the like) and the small ones are just a shelled snack for the Cichlids (BTW, I like Cories, and I apologise to my four for the bluntness which they have just seen on my screen here...) I would lean more toward a Clown Loach or some other large aggressive loach. Be diligent here, too, because your Plec might have other thoughts about allowing another bottom-dweller into his realm.
Finally, good eye with the so-called Assorted South American tank--most would not think to look their for Firemouths and Cons, as they are most definitely Central American in origin.
Best of luck with the tank!
So the two I like I'll put in my 150 when they grow a little. I just might keep them in the same tank. The wife really likes the convicts when she saw them. She said I can get another tank to keep them in.