Hi in answer to your question
A lot of fish in nature use other fish around them to put them at ease, to show there's no predators about, etc. In the aquarium as in the wild, many fish will cower in corners or stay hidden all the time if they don't have other fish swimming about signaling to them that things are ok. So the basic theory behind using dither fish is putting fish in the tank to make the other fish more at ease.
This idea works especially well with cichlids. Silver Dollar's (not Tinfoil's) are a large 'dither' fish and peaceful making them a great dither fish. They are also SA origin and get to about 6-7 inches. Get a school of 6. If you get very small dither fish, they may be eaten!!
Catfish can be a problem as they are also bottom dwellers and can upset cichlid territories, especially when spawning. Also most cats are nocturnal scavengers/hunters, whilst most cichlids are diurnal. This can mean your cichlids are going to have a lot of sleepless nights!!
Dwarf Rams such as Microgeophagus Ramirezi. The red paradisefish, Macropodus opercularis, for both the more aggressive dwarf cichlids and medium sized cichlids. Also called a chinese Fighting Fish, not Sa or CA but will add some striking colours. Just do a bit of Google searching for other ideas.
Hope this helps