Ok, I feel comfortable answering this question being that I have bred the same 2 pairs for some time now and have thousands of fry that survived to become food for my main tank.
Ok with that said, there are a few things lacking in your setup for a pair to successfully breed. First of all, you need some sort of substrate if they are a stubborn our that won't breed. I breed angels in bare bottom tanks but then again, you aren't successfully breeding. Convicts love clearing the substrate in the spawning area.
The second missing part would be heat. If you normally keep your temp in the mid 70's it needs to be raised. I keep mine at 80-82f. Also if your fish are stubborn, 3 large water changes 3 days in a row which simulates rainfall(breeding time in nature) will stimulate spawning.
The size of the fish is really of no concern as both the males and females can breed at around 1-2" and reach sexual maturity very young.
As others have stated, regarding "dither fish", which is using another convict or similarly colored or shaped fish to incite the parental instinct, is not needed. Convicts will spawn without these additions.
As far as pairing is concerned, if you have a male and female, you have a pair. They are not exactly picky lovers lol.
I don't know if you use a timer for your lights, but is always smart to keep your lighting schedule very consistent.
If you want to breed and raise convicts, it is not necessary to remove the eggs or fry for artificial incubation. In most cases an inexperienced breeder will have much more luck letting the parents care for them.
If you have any other questions let me know...
I can also post pictures of my breeders and fry if you need evidence that I know these lil guys lol.