You need to look at the ADULT size of the fish for one, as previously explained the one's you do already have will outgrow your tank and should be moved to a larger tank, this will not be sufficient for them little whiles down the Rd.
Secondly you have to look at a tanks volume (gallon) and its bio-load (fishpoop), also here you're maxing it out as it is. Not having any plants in the tank make the matter even worst for the tank's ecosystem to try handle this load.
So no adding another small or big fish just doesn't work in the long run.
You can add them sure, but then be prepared they will die on you far quicker then they naturally should.
The tank will already be past its capacity with the Sword's in it being male & female, they'll have you baby's in there like NOTHING so the only thing there to not get more overstocked then it is, is to hope the Angels are large enough already to eat them all as they're born.
Sorry to be so strait fwd, and I know it ain't what you want to hear, but that's just how it is.