From what I've always been told and what I read about angelfish (+ my own experiences, one of my favorite genusses of fish, if not my favorite) there are 3 main families.
Pterophyllum Leopoldi: The smallest of the 3, about 10cm (4") in length and 20cm (near 8") in height. Over where I live (the Netherlands), these are pretty rare to find.
P. Scalare: Mostcommon one of the 3 and get about 15cm (6") in length and about 25cm (9.8") in height. Depending on the quality of the fish, you might get one with larger or shorter fins, making the fish longer or shorter in height (I had one that was stunted in body growth, yet still reached 22cm (8.6") in height because he had long fins. Some great examples of wild forms in my opinion are: P. Scalare Manacapuru Red Back, P. Scalare sp. Rio Nanay, P. Scalare and the P. Scalare "Guyana Red Dragon".
P. Altum: The biggest and tallest. Would highly recommend a tank with a
water height of 80cm or more (31"+) because these guys get tall, about 35cm tall (13.7") and 20cm (near 8") in length. They are magnificent fish and also very calm eaters in comparison to the other angels. They're said to be harder to keep than P. Scalare and Leopoldi, but I have no personal experience with keeping P. Altum, so I can't confirm this.
When looking for young fish that aren't yet grown out yet. Look at the eye to body ratio, this is the best way to spot an angel with stunted growth. From a young boy on I've been told to image a vertical line on the angel's eye in my head, and for a "healthy" one you'd have place for at least an eye and a half above the eye of the fish and one and a half to two below the eye of the fish. Then you'd have one that has had optimal growth up until you get the fish.
If you're planning on growing a group out from tiny babies that are like 2cm tall (1") it's definitely advised to feed 5 times or more a day, making sure the fish never has an empty stomach during lights on, and to do daily 50%+ water changes. If you're picking Semi-Adults, most of the "hassle" will already be done, but here locally they rarely have semi adults.
Anyways, hope you'll enjoy your angelfish
I've grown up with angels and they're definitely a very social fish. I miss my angels, such great characters and always begging for your attention (and food...).
Edit: Also, most angelfish will eat anything that fits in their mouths. from shrimp to the smaller Tetra species (I've seen Scalares and Leopoldi hunt adult, Cardinal Tetras). Getting them as small juveniles or as babys will definitely make it less likely for them to eat them, but it will not guarantee they won't be seen as food (either constantly or only periodically when they're in a "hunting" kind of mood).
