No. An UNI can never stack on a Blue Bomber (or another UNI) as they are the same shape. Additionally a Blue Bomber can never be played on an UNI (or another Blue Bomber).

It depends.

If a layer still has playable spaces and there are less than 6 pieces on that layer, you cannot start a new layer and must play on the current layer. If you can't make a play on that layer because you don't have the right game piece to play, then you are out of the round.

If there are less than 6 pieces on the layer, but no legal plays to make, you may start a new layer.
For example if a TRIO and a QUAD is covering all available spaces below, there are no spaces available for that layer and therefore no legal plays left. Even though there are less than 6 pieces played, a new layer may be started.