[Cazador]

Most of the time, two different species wouldn't even attempt to breed because of anatomical, behavioral, temporal, or geographic differences. Even if two different species actually copulate, formation of an embryo is extremely unlikely because of differences in fertilization mechanisms. If the sperm manages to penetrate the egg, it is then highly unlikely that they would have the same number/type of chromosomes. The number and type must match... 1/2 from the male and the same matching 1/2 from the female. The actual genes on the chromosomes pair up, and if genes are missing, kinks form on the chromosome that has too many genes. These misshapen chromosomes, then, don't replicate properly, and the embryo doesn't develop. If, however, there are only very minor differences, the embryo might continue to develop and either 1) die or during before birth, or 2) be born with reduced vigor/fitness. It is extremely rare that two different species ever produce offspring with even limited viability.

Mules are the classic exception. As I recall, the mother is a female horse, and the father is a male pony. The offspring (mules) are normally sterile. Once in a great while, fertile mules are born, but this too is due to mutated mutations.

After all this, the short answer is that ribbons and garters might be able to breed (particularily in captivity where competition is limited, and they're confined), but it's highly unlikely that they would produce viable offspring.