It has always been a common practice to put 6 or more smaller Angels in a tank and let them grow and pair off naturally. I try to get "Young Breeder" sized fish so I don;t have to wait so long. When I am looking for fish for a display tank, I use small-medium size but for intentional breeders, the larger the better. Young breeders will probably be about 1/2 dollar in body size making them about 6-8 months old. This is when they start becoming sexually mature. Obviously, the larger the fish, the less you can put in a 30 gal tank.
The issues you may be having with the 2 larger fish is that they were forced together. This does not mean they won;t ever pair up. I have a pair of fish now that I forced and when I first put the female into the male's tank, he beat her up. That lasted for a day.
She kicked his butt the next day and continued to do so even when she decided to spawn. He just ate the eggs and she just beat him into a corner. I put some decorations in the tank to separate them for a few days and once I removed them, they both calmed down. Then she spawned again and he started eating them again only to stop after a few lines and he started fertilizing the eggs instead. I pulled the spawn and now have their fry. So it's all about understanding the fish. I took a supposed female, who kept being hassled by every male I put her with, and decided to try her with a larger male that had himself been beaten by a female. They were fine together for a few weeks but then, when he was ready to spawn and she apparently wasn't, even he started beating on "her". So I am having serious doubts as to her gender even tho she looks like a female. My plan for her is to put her into a community tank of adult Angels and let her pick her own mate as I am not having much success at it. And they call me " The angelfish Love Dr."
So you see, it's a little bit science, a little bit knowledge and a little bit luck. I just try to reduce my need for the luck.
Having more fish for your fish to choose from reduces the need for luck.