I have a single, fullscreen SurfaceView. And I have multiple network streams with h264 video which I can decode using MediaCodec. Is it possible to specify to which coordinates of the Surface will the video be rendered? So I can create kind of video mozaic?
No, that's not possible. You'll need to use multiple SurfaceTextures instead, one per video decoder, and render all the textures into one view using Open GL.
See https://source.android.com/devices/graphics/architecture.html for more explanations on how this works; in particular, each Surface can only have one producer and one consumer.