You can breed ditto's with male Pokemon, you will get a baby of the lowest evolutionary form of the pokemon that isn't ditto (that is why you cannot get a non-hacked ditto caught in Diamond/Pearl that is below level 29), if you are breeding without a ditto, eg Swampert (Male) + Blastoise (Female), the pokemon in the egg will be in the evolution line of the mother (eg Blastoise)
You only will not get the lowest level of the evolutionary line if the parent is Wobbufet, Maril etc (there are a few others I believe) and not holding the right item.
Did that help in any way?