It is my experience that small flames are juveniles and not yet sexually mature and are typically about a year old. They used to be caught in the grass flats in Hawaii in large numbers but now I understand that most flames come from a non-U.S. island. After passing through their juvenile stage I believe that they are all female and will stay that way the rest of their life until an opening or opportunity comes up within the harem, such as the dominant harem male dying off or disappearing. Then typically the largest female then changes sex in just a few weeks into a male. Even though angels are taken from the harem during capture and transport, it is my guess that they typically don't all "turn male" during the relatively few weeks of capture. I think most of the decent sized ones will still be female when you see them at the pet store, although the largest are likely males. I don't know how long they need to be isolated until they change into males.
I don't know if flames have ability to change back from male to female although it has been reported among a few dwarf angelfish species. I tend to doubt it, but would be happy to be proven wrong.
Regarding tank height, 18" is probably adequate but in my opinion a much deeper tank is better. The drive to procreate is very strong but we need to help these little fish by not restricting the height of their spawning rush. My own (unfinished) tank is probably overkill at 37" but I think it is better to err on the side of caution.
Also, Oceanic Instituted found an increase in their eqq quantity and quality with large tanks, up to 500 gallons as I recall. I'e heard of two theories here. One theory is that, without doubt, the best way to culture larvae begins with high quality eggs that come from happy and well fed parents with adequate living space in high quality water.
But the opposite theory also appears to be true, that angels in small tanks (with adequate height) will spawn better if they have limited room and limited distractions. So I hear that angelfish breeders have been successful with 55 gallon drums divided into two vertical halves, without live rock or much tank decoration.
Which theory you follow depends on you, and they have a lot in common. We hobbyists tend to have tanks smaller than 500 gallons so we are probably forced to use the second theory. Then again the sex drive is so strong that it would tend to overcome many obstacles. Put two mature fish together and we'd be surprised at the lengths they will go to spawn.
Regarding the parvocalanus copepods, that is what the Hawaii folks found to work as a first food, but once the copepod nauplii get to a certain size they escape predation by the angelfish larvae. Congratulations to anyone who has gotten larvae to 22 days, it proves you have overcome the obstacle of an appropriate second food after the parvocalanus. Or you have provided parvo in such density that some vulnerable nauplii have been available to your baby fish.
I hope that helps, and keep up the good work.