Is it ok to use pudding as a cake filling? The ones that you just add milk.
The only problem with filling is that it might be so soft that it would squeeze out the sides.
I've done that but it can be very squishy and can cause bulging and sliding of layers. You might want to use less milk and make it thicker than usual. And definitely use a wide icing dam around the edge of the cake to keep the pudding in.