Small Angels do better in schools while larger Angels can do just fine alone. I suggest, if you see the fish staying in the back or acting overly shy and not feeding, I would rehome the Angel to a tank where more can be kept together.
There is another issue you may need to deal with however, if you add another Angel. Angels live in a hierarchy society so the more you have together, the less pressure any one fish will get from the "Leader" of the pack. The less you keep together, the more focus gets put onto the few other fish and this can cause damage to the other Angels. 2 Angels, that are not a mated pair, have just as much chance of beating each other as they do living peacefully with each other. Every Angel is different so there is no way to tell what YOUR outcome will be. It's, unfortunately, a "try it and see" scenario. That's just the nature of this fish.
Hope this helps