You can mix Jetstream and Xstream second stages on the same Xstream first stage, as these stages all work with the same intermediate pressure. But you can NOT mix Jetstream or Xstream stages with Cyklon stages. The reason is that the Cyklon regulators work on a higher intermediate pressure than the Jetstream and Xstream regulators.

Also, Poseidon has recently introduced the Cyklon X regulator which consists of one or more Cyklon second stages together with a version of the Xstream first stage which is configured for the higher IP required by the Cyklon. The Cyklon X first stage is similar in appearance to the Xstream first stage but etched with "Cyklon X 0110-006" on the bottom. The Jetstream and Xstream second stages must not be connected to a Cyklon X first stage, nor should Cykon second stages be connected to an Xstream first stage.