I havent really ran into any special technique for making the samples fit in in a harmonic pattern. Sometimes it just does, other times it just dosent blend. Try using sounds that are similar. As far as time strectching and pitch bending, it all depends on weather your samples are loops, or are they hits, ore maybe a 1 or 2 bar length sample. The shorter the sound, the easier it is to pitch bend. If your trying to pich bend a whole 2 or 4 bar loop, its going to be difficult to get the right sound that your ear wants to hear.