I wouldn't consider them a peaceful species. They're semi-aggressive to aggressive. Matured males can be very aggressive towards each other and possibly to matured males of other nimbochromis species.
With that said, Rift lake african cichlid temperment and aggression is generally in relation to what they're kept with.
Mbuna's (which they probably shouldn't be kept with) are generally more aggressive than venustus. If kept with them the mbuna's are capable of dominating them despite being smaller. Venustus probably won't seem aggressive at all if kept with them.
When kept with other lake malawi 'haps' (their most compatible tank mates), venustus the can be a little on the dominant side. Particularly the males....and especially the males if their female counterparts are also in the tank.
I've kept them with malawi eye biters and malawi blue dolphins and the male venustus was the alpha fish in the tank. The females were generally peaceful, though as mothers they would get VERY protective when their fry were concerned. I always knew that one of the females spat fry in my 180gal when all of the other fish in the tank were huddled in one half of the tank, while she occupied the other half (with her fry congregating in one of the tank corner's on her side of the tank). (this would only last for a day or so).