Animals can breed inter-specially when they have a recent common ancestor. For example, lions and tigers have a recent common ancestor that was very similar to them, and therefore they can breed. The same thing goes for horses, donkeys, zebras, and other equines.
Now all [non-Legendary] Pokémon are apparently descendants of Mew. Chronologically, below Mew are the Ancient Pokémon: Aerodactyl, Relicanth, Kabutops, Lileep, etc... [at least] one for every Egg Group that exists today. Below these ancient Pokémon are the current 400+ that we know and love today.
Considering that there have not been any reports of a fish Pokémon being evolutionarily placed between Magikarp and Relicanth, all Pokémon have one direct, recent, common ancestor: a fossil Pokémon. This makes them close enough to breed interspecially. However, the distance between Egg Groups is further, as the differing Group's common ancestor, Mew, is too far away.
So in scientific nomenclature, while Cradily would be something like "Poképlantae Cirripedia", Vileplume would be "Poképlantae Rafflesia". Same genus, different species, but related closely enough to breed with other members of Plantae (like Sunflora, which would be something along the lines of Poképlantae Helianthus).