Again I must disagree with
@mr_tap_water on several points,
Yes, it is possible to bend the rules a bit and do some things that are not generally recommended, but in this case we are talking about a tank less than half the recommended size for a flame angel.
There are also so many other possible options in fish selection that you almost always can find something else. Here are a few I would consider in place of a flame angel. Plenty to choose from,
Any of the fire gobies. Make sure your tank is well covered, as they jump.
Most other small gobies.
royal gramma - Just about the perfect reef fish
Pajama cardinal
Banggai Cardinalf
Neon dottyback - These can be aggressive, add last
Yellowhead jawfish - Has some special requirements
Blue spot jawfish - Has some special requirements, expensive
Spotted hawkfish - one of my personal favorites
Most other small hawk fish, flame, longnose, or arc eye and so on
As for the textbook being out of date, with such resources as the internet, up to date information is easily available. Obviously if I or anyone else was quoting from a book from the 1980's or something, it would be obsolete.
As for taking a fish back, or having it for a short while, it's a massive job to remove a fish from a reef system without doing a lot of other damage. Sometimes you get lucky with a trap of some sort, but often it means removing most of the rock to be able to remove a fish.
If you are going to be in a position where you are going to return a fish, check the store policy. Some don't accept returns at all, some will give you a store credit, some charge you a restocking fee. Be sure to check the store policy before you purchase.
One additional thought for the op. Don't get upset because someone is telling you that this is a bad idea. Don't get your heart set on a single fish. I once say several sohal tangs in a very large tank in a public aquarium. It's a fantastic looking fish, and I though I would love to have one in my tank. After checking I found that they can get extremely aggressive and some individuals reach the point where they will kill anything else you want to keep with them. As much as I wanted the fish, I had to pass on it.