@John > There are already 4 decks on the DDJ-SX so you could plug in your two CDJ900s as auxiliary inputs and use the mixer in standalone mode. This would mean you could have a total of six decks, 4 for playing and 2 backups (or you could use them to play CDs / USBs etc).
The Serato DJ software won't be available until November 1st.
Pioneer does not make any announcements about products that may be in the pipeline.