Oh the land of the -rather large- Arapaimas! How fun was your trip!? And I do think that those are Silvers, they fit the bill with loci, size/color. Nice you were able to fish as the gov't has been not granting permits for fishing as of late I hear.
My trip was very fun. If you know some locals, good things will happen. I always fished with locals, and we always threw the fish back. I don't know what the government regulations are, but usually we had a local lawyer with us...
The top one certainly looks like a Centropomid (a type of snook) but the dorsal fin is connected which would not be correct for the family. I think it is a type of drum (there is a local freshwater variety called a Basha).
The bottom one is a Pimelodellid catfish like Matt said.
Come to this month's GWAPA meeting where I will be one of the presenters. In a nutshell, however, a good friend of mine knew somebody down there, so we went to look at fish and to experience an exciting tropical country where not many tourists go.