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[cockroach]

I would look into a school of anything that is NOT flashy sliver. That is a trigger for attacking.

You could look at non "traditional" or non "trendy" dithers such as
- Clown Loaches: they grow slow so you would have to start with a decent group of good sized fish so they don't become snacks. This will be pricey though andcan be a 20 years + commitment.
- Maroon Sharks: Leptobarbus hoevenii. Even though silver, they are not that reflective "enticing" sheen. They grow large and apparently school.
- Along the same lines is the red finned cigar barbs: L. rubripinna
- Red Hook Silver Dollars: Cost some dosh $$$.
- Masheers: Perhaps one of the medium sized species could work.

Spit-balling ideas
- Featherfin catfish: S. euruptus. When well cared for they can grow some size. And don;t mind being in groups when they have the space. Will also fill an area in the tank not taken up by the dorado.
- Pictus sp.: look at one of the species reaching 25-30cm and get a group.
- Spanner/T-Barbs?: could these work? The reach about 6 inches long and 3-4 inches high.
- anostomus/leporinus: Some of these get larger and are fast. They are also not shiny or flash.

 

[cockroach]

You will have to drop the schooling fish and the piranha for this to have any chance of working out long term. The ATF and dorado will make quick work of the 'schooling fish'.

You could always look at adding more oddball specimens to jazz up the tank:
Hi-fin sharks: Myxocyprinus asiaticus
Dats
Paraya: Hydrolycus scomberoides (although have read on here, people struggle to keep them alive after a certain size.)
Knifefish - e.g. clown

 

[RADNP]

I just thought about this again, and sort of expected the responses that it might not work out with that combination, but when it comes to the lone piranha, would the piranha be the one in danger, or would it be the threat?