Though, in my Pichu's name, we stress the first syllable, not the second - like in Peechu. And I think Pikachu is like Pikachoo, isn't it?
In pokemon Pikachu and Pichu are two different pokemon. But when Pichu grows up he turns into Pikachu. The way you pronounced both Pikachu and Pichu are the same way these two different Pokemon are pronounced. LOL can you tell I spend way too much time watching cartoons.