Pokemon eggs hatch into level 1 Pokemon, so if its a level 1 breedable Pokemon (like the Piplup), then its fine. There is no level 1 Giratina to my knowledge, and Ambipon is an evolution that requires a level to be gained, so those are hacks. Its also possible to get level 1 Pokemon like Jolteon, because it only requires a stone to evolve. Just use your judgment on whether it looks possible or not, and be aware that there's plenty of hacks floating around on the GTS.
Trading a hack into your game shouldn't do anything to it, so don't worry about it.