Do I have to keep every speak under Bose for the whole system to work? Or can I purchase different brand speakers?

 

Differing opinions now how it ends up as the Bose speakers of have unusual impedances that are custom matched to the Bose amp. In a nutshell I think you might need 2ohm speakers or it will not play very loud but I could be wrong. You would need to do some searching as many others have tackled the entire scenario ranging from replacing just the stereo to speakers, to bypassing the stock Bose amp (tricky since all the speakers go to that, not the head unit in the dash).

 

You can read this:
https://www.roadandtrack.com/car-cul...o-development/

Or, match your replacement speaker(s) with oem specs.