Howdy,
It is a common misconception that Pantodon buchholzi does not need much room. Although they seem passive most of the time, they "wake up" when live food hits the surface. A 20 gal will be too small to allow this spectacular natural feeding behavor to develop. A 55 gal will be well suited for a pair. Toss a cricket in the tank and you'll see what I mean...
The knife gets big, so you may even want to think about a future upgrade for a 55 gal.
Congos are beautiful fish, but they will eat any plants you may choose to incorporate in your set-up. Especially the butterflies will appreciate some surface cover (either Vallisneria or floating plants)
BTW: I think theme tanks ("Africa") are great
HarleyK