If you want to breed swallow tail, ribbon tail, longfin, or lyretail livebearers and you want 100% that strain fry, you have to use AI. Forgot where I got this from. Never done it but should work... if I can get my hands on lyre swords I can tell you how it works!
Clove oil your male fish and hold it belly up in a piece of wet tissue. Move the gonopodium back and forth a few times. Starting at the head, rub the fish downwards to the base of the gonopodium (no detail given on how this was done). Fish releases milt - collect it in a pipette and place it in a drop of saline, and put the male in a recovery container.
Clove oil the female, hold her the same way. Using a 0.5mm Pasteur pipette (can be created by pulling a glass pipette over a gas lamp, breaking to required length and sandpapering) insert the saline containing the milt into her genital opening - use four or five drops. Put her in a recovery container.
Supposedly she is now pregnant. This has to be done because the male's gonopodium is an awkward shape - this is why he cannot mate and also why these genes are never seen in the wild.