Reply to Studio Question! Help.

Hey,
So were building a studio! so excited.
We now have a set of KRK Rokit 8.
Were getting a set of Yamaha hs80m.
Also possibly getting a sub - Recommendations? (Is a sub necessary?) The room is about 10 by 8.

Main question:
What is the item which is needed with multiple inputs to select which speakers are being used when? Is it an audio interface? Or what.
Recommendations needed again.

Thanks,
Jed.