OK, here we go. I used to breed Angelfish and can relate very well to your questions. First off, Angelfish are a South American Cichlid. That being the case, you will probably notice they can be very territorial. All Cichlids are aggressive; the Africans being the most aggressive. The larger they become, the more aggressive they will be; specially if breeding. Determining the sex of an Angelfish is difficult and usually a group of six will produce one mated pair if you are lucky. In a 40 gallon aquarium you should hope that the two you have wind up a mated pair. I would not introduce anymore unless you go to a larger aquarium. with plenty of plants for hiding, and even then I would avoid it. Angels live about 5 years if healthy and grow to a pretty good size. When trying to add other fish to the aquarium you can take the following into consideration. First, because of the long fins on your Angels, avoid adding fish from the Barb and Tetra families. They are both fin nippers and usually pretty quick fish. They will probably terrorize your Angels until they are caught and maybe killed. Below in the reference section you will find further info about community fish. Remember, Angels are going to be pretty aggressive toward most fish. The Bristlenose Plecostamus and others from the Plecostamus would be fine as would most catfish or Algae eaters, as well as other bottom feeders such as the sedontis family. I personally would not add Mollies, Platys, Guppys, small tetras or other docile community fish to the aquarium. Anyway take a look at the references and go from there. Hope this helps. Remember, the previous information is only based on my prsonal exxperiences. There are always exceptions! Use this linkhttp://www.ratemyfishtank.com/freshwater_compatibility_chart.php it is a compatability chart.

You are correct that Angel Fish can be aggressive to smaller fish or even fish of the same species so it is important to keep small fish out of their tank. It is usually a good idea to have multiple Angel Fish together, typically 4 to 6 in a group, because Angel Fish will often taunt and torment the weaker fish if there are only 2 or 3 in the tank. A larger number of fish allows the smaller Angel Fish to feel more secure and live a happier life.

As for other fish that can live happily with Angel Fish, there are a few. Firstly, it is important to remember that your Angel Fish should never be housed with small fish, including Neon Tatras or Guppies as your Angel Fish will likely see them as food. Some fish that your Angel Fish can live happily with are Bigger Tetras, Silver Dollars, Corydoras Catfish, and Bristlenose Catfish. You can also try various livebearers, including Platies, Swordtails, and Mollies. Generally you should be looking for other tropical fish from roughly the same region as the Angel Fish, that being South America.

These are some fishes that are compartible with the angel fish.
Silver Dollars,

http://z.about.com/d/freshaquarium/1/0/X/B/silver01.jpg
Bigger Tetras such as Black Skirts, Serpaes, and Silver Tips,
http://www.tropicalfishandaquariums.com/Tetras/BlackSkirtTetra2.jpg

Corydoras Catfish, Livebearers such as Platies, Swordtails, and Mollies, but not Guppies which may be eaten by Angels, and one Plecostomus Catfish.
http://exotic-aquariums.com/images/platies.jpg

Angel Fish are often seen in aquariums with Gouramis, Giant Danios, Rainbows, a group of Bala Sharks, and a group of Clown Loaches.
http://thesmileyfish.com/wp-content/uploads/2008/03/tropicalfish005.jpg