The Flex implants are flexible, to a degree. Long term flexing/momement is likely to damage them. P0 undergoes a lot of movement on a daily bases. That, coupled with the size of the implant (increasing the chances that you’ll feel it), makes P0 an undesirable location. P1-4 undergoes significantly less movement, making it a better fit for the flex implants.
Parallel to the index finger places the flex implant in an area with less movement compared to between the thumb and index finger. (It’s closer to P1 than P0) So you might be able to get away with it there, but you’re better off with P1-4 like @Pilgrimsmaster said.
Ps. Welcome to the community 