@Christoffer Földes > Congrats on the gear mate, you're gonna love it! Yes you should connect your CDJ-2000's and your DJM-900 to get the most out of the setup. This enables playing with Rekordbox from your computer, playing multiple CDJ's from 1 USB device or SD card, and quantized effects.
The straight forward reason as to why you need a switch is because there isn't enough ports on the DJM-900. ;) lol But there is not a lot of room available on the DJM-900 as it is and couldn't fit another 3 LAN ports properly, let alone it being costly and would have increased the price of the mixer.
That being said, I use the D-LINK DES-1008e (8-port) switch which is appealing to the eye, wasn't very pricy, and works perfect for me. D-Link also makes a DES-1005e (5-port) switch which is less costly and works just as well.
In all honesty, it's really only 2 more cables added to the set. And you can use digital cables from your 2000's to your 900 which cleans up all the RCA cables. I like the digital inputs and outputs! But if you decide to get some digital cables, make sure they are 75ohm for best quality.
I hope this helps.