Lets say, could you put an Eevee and a Ditto in the daycare and get an Eevee egg? I think I've tried it before, but I'm too lazy to try again.
As long as the Pokemon isn't in the Undiscovered Egg Group, it will be able to breed.
Any Pokémon that is not in the Undiscovered egg group can breed with any other Pokémon within its own egg group. Do note that 'baby' Pokémon (Azurill, Riolu, etc) can't breed, but will always have three flawless Individual Values (IVs) which will be useful for breeding when they do evolve.
It doesn't matter if the Pokemon is evolved or not. The only way to breed is from breeding two from the same egg group. Legendaries can't breed. There are a few Pokemon that can only breed with ditto such as porygon and metacross.
I hoped this helped