I have always had angels in pairs.Sometimes same sex pairs,sometimes M and F.Right now I have a pair in a 30,with dwarf cichlids,otos,and cories.The angels are the same sex(females,i think).From my experience with them,as far as aggression,even when they are a little territorial,what aggressive behavior they do show,never does any damage to the other fish.They mostly just chase one another around the tank.Sometimes they go after the dwarfs,but like with each other,no damage is ever done.So if you do decide to get some in your 40g,I would stick with 2.
Of all the angels I have had over the years,mating pairs,and non mating pairs,non has never done any real damage to another fish.They have a reputation of being aggressive because they are territorial.But in a 40,just don't overstock it with other large fish,and you should be fine.