Not true, none of the smaller species like Channa gachua or Channa bleheri (the two most commonly seen dwarf species) could eat a adult convict Cichlid and would in fact be in danger of being killed by the convict Cichlid/s which are far more aggressive.
Personally i am of the belief that snakeheads are best kept in species tanks, they preffer it that way and you get the best from the fish. I'd go for a single C.gachua or bleheri in a 30x12x15" and heavily plant the tank or go larger with a 48x18x18" and keep a group of 6-8 again in a heavily planted tank, if you have a pair form then they could be kept as a breeding pair in a 30x12x15".
Single dwarf snakeheads also can work in communities of larger peacefull fish like bala sharks, larger gouarmies like snakeskins, angel fish and basicly anything that has a tall body shape so cant be swallowed. Dont keep them with anything aggressive as the snakehead will usually come off worse, although of course this only applies to dwarf snakeheads. The exception to this with smaller species is Parachanna obscura which in my experience is insanely aggressive and will not tolerate tankmates of any kind.
For the larger species solitary species tank care is the only way to go unless you are lucky enough to have a breeding pair and a tank large enough to house two large snakeheads, large snakeheads (especially red snakeheads, Channa microplets) will attack and eat anything they share a tank with eventually.