Congos.
Congos tend to be on the tougher end of the tetra scale.
They will fin nip when give the chance.
They would go nicely with some of the other tougher tetras like:
Black Skirts
Serpaes
Diamond tetras
Columbia tetras
They would also work with some of the more doscile barb species.
Cherry barbs
rosey barbs
arulius barbs
Spanner / t-bar barbs
I wouldn't personally put them with anything that has long flowing type finnage, or with anything that tends to be on the weaker or nastier end of the spectrum. IE:
Angels
gouramis either too doscile (Moonlights)or too tough (Opaline, kissing)
Discus (they just stress too easy, losing a $50 discus to a $2 tetra is just not worth it IMO)
Neons
cardinals
I tend to err on the side of caution though.