I already have a pH regulator, can it work with BoPi?

Yes, if its not already controlled by the pump, it can be controlled by BoPi and can be turned on with the pump, with or without a delay.

BoPi can’t however get the pH value from that external regulator. To get the pH value in the app, you will need to add another pH sensor. It is not necessary for correct operation of BoPi or the pool and could be added at anytime.