Using KERS will get you kicked out of FOTA.
FOTA has all teams in it now, so being kicked out would be particularly bad for a team as they would be outnumber in any decision making.
Altho, If FOTA starts to look like a waste of time and like it might fall apart, I wouldnt be surprised if some of the teams 'conveniently' has KERS setups they were able to just drop in.
I don't know if the cars will have space for the system. This years McLaren has smaller sidepods than last years and contains a fuel tank twice the size.