Silver dollars should be left alone if they are big enough not to be viewed as food.I don't see them causing the payara any problems...not sure about Arowana.
When I was still keeping payaras, I had mine with some ATF, silver dollars, barbs, arowanas, catfish, etc. Anything that was too big to be eaten was essentially ignored. I've kept tats and scombs and both of those species seem to be completely non aggressive, just predatory.
The Melanopterus species of Brycon (the one with bottom half of the tail in a black stripe) work well with other fish. I've had 2 for a while and they seem very peaceful.
The Brycon Dentex are supposed to be aurnry. I lost mine to rays within a few days so can't confirm or deny. Others report them being a handful. I wouldn't think it to be worth it, there's better tank mates out there....
I have 2 8" armatus with a bunch of bichirs with no problems and 3 5" scombs with 2 hujeta gars, 1 African brown knife and 1 aba aba knife with no problems as well.