If I were doing a planted tank with a South American theme - I'd keep a pair of Bolivian or German rams, with probably 10-12 neons in there for a bit more "life".
If I wanted something else though - I would go with about 5 multis, with a sand bed and plenty of snail shells.
Personally - I am not sure which I'd rather have. Screw it, what I would do is just rack two of the tanks, then keep two 2.5 or 5 gallons beneath them to run as sumps.
Monster fish are fun, but so are the smaller fish!
Drew