Depends on what kind of cichlids you get. If you get a gold severum, blood red parrot, geophagus and cichlids of that nature, a black ghost knifefish would work well with them in a 125. I wouldn't get a CK, though, with that size tank and I would not put a CK or BGK with very aggressive cichlids like the texas, red devil, etc.
How would you feel about a setup like:
1x Black Ghost Knife
1x Gold Severum (or red-streaked Severum)
1x Geophagus
1x Blood-Red Parrot
6x Clown Loaches
12x Roseline Sharks
That would fill out a 125 well I would think. The BGK can get up in the 20-24 inch range, but it takes them years to get there. A 125 can house one for a very long time.