You only have a 55 gallon so I wouldn't really consider getting more than two.
That tank is really only fit for one, but if you want two, the driftwood should help.
Senegals are a smaller species and several should be fine in a 55, assuming bioload is managed with water changes.
As far as aggression, I had two ornates and for awhile after adding them one used to drag the other around by the tail, I'm guessing as a dominance tactic or something. Eventually he got over it and stopped though. My ropes also used to bicker despite having several. The biggest one actually put one of the others head in his mouth once, but no injury as far as I could see.