DDJ-SX2 With CDJs

I'm considering buying a Pioneer DDJ-SX2. Since it has 4 decks, but two jogwheels, I'm thinking of adding two CDJ (or XDJs) to make it easier. They should have the same functionality as the decks on the SX2, so I can play tracks from SDJ on them, without it being necessary to put the library on USB sticks. Even though it is possible for me to do that, and I will if I need to, I'd rather just have my laptop music library routed to the CDJ. So, two questions:

1. Can I connect a CDJ to the DDJ-SX2? (Keep in mind I'm using Serato DJ)

2. What CDJ is the most similar to the layout of the SX2? I'm on a budget, so it shouldn't be too expensive.

Criteria for CDJs and/or XDJs:

1. Must have usb input

2. CD input is optional

3. Able to play at least both MP3s and WAV files.

4. Preferably has a large tempo control (+/-100). This isn't required, though, +/-16 is enough.

5. 4-line display is fine, though waveforms are preferred. Again, this depends on whether I can sync SDJ with the CDJs.

Thanks for the help!