It's not so much a size difference as a situation of territory. If the existing angels are mature enough to have established territory in the tank, there could be a problem. This is less of a problem if it is a larger tank, or a larger group of fish. Small angels work out fine with adults, as long as the smaller ones are too big to eat.
The best way to introduce the new fish is to take the existing fish out of the tank, keep them in a bucket for a couple of hours. During this time rearrange the tank, and introduce the new fish. After a couple of hours put the previous residents in the rearranged tank with the new fish. This removes the old territories, as fish remember established territories off of objects in the tank for the most part. You want to make the tank as different as possible from the way it used to look. Don't worry if it looks goofy, or totally in disarray, you can move it back to the way it was in a week. By then the fish won't remember the way it originally was.
As always, quar the new fish.
