some fish do better in pairs, some preer larger groups. Others like to have dither fish in the tank with em (dither fish, are like having a crapload of tetra's in a freshwater discus tank, the bigger nicer shy fish see all the tetra's swimming around and assume it's safe to be out and about, when the dither fish run for cover, it lets the larger fish know somethings up). But as to weather or not the flame angel needs a companion, I couldn't say.
I do know that it is not recommended to keep more than one pigmy angle in a tank together, since they can be teritorial and aggressive to one another. This would make me think that the single specimen you have should be fine on his own. Since you shouldn't add another flame angel to that tank, and i really don't think he's going want any other fish encroaching on his turf, I would hold off on more fish if that's what you are wanting to do anyways. He's not going to die of lonelyness in the meantime.
