The common ancistrus in the hobby comes from the Amazon, the parana basin I believe.
Biotope correct fish for firemouths are not always easy to find unless you look at livebearers.
Tetras like astyanax mexicana can be found with them as can the pike top livebearer. The pike top is an interesting fish but can be difficult to feed,preferring live foods.
Catfish are rhamdia species but may get too big for your tank.
There are also sturisoma species in central America.