As The Reverend says, it's difficult to make an informed recommendation without knowing what else is in the tank.
On the face of it, Endlers fit the bill, in my experience guppies can be a bit boisterous on their own. ALthough they're not a shaoling fish as such, their boisterousness is contained to themselves when they're in a group, but can be directed to other species if solitary. Endlers on the other hand don't have that same level of boisterous.