You cannot change your statistics when shapechanging
The Doppelganger’s Shapechange feature states:
[…] Its statistics, other than its size, are the same in each form. […]
And the Changeling’s Shapechanger feature states:
[…] none of your game statistics change. […]
We know “What all is included in a creature’s game statistics?” which is basically everything about an NPC, monster, PC, or anything else really. And these features cannot change your statistics. So while you might be able to look like a hybrid form that you have seen, you would not gain any benefits that that creature usually has.