Besides plus and minus 12V, P8 also supplies the POWER GOOD signal from the PSU. Without the POWER GOOD signal, the motherboard won't start. Obviously, because your 5155 is now booting to BASIC, you must have reconnected P8.
Yes, it it quite okay to have P8 plugged in without C56 being present. Ideally, you should fit a new C56 later.
Hmmm... I only have P9 plugged in and it's booting to BASIC. I'm still getting the same problem with the power when I plug the P8 connector in. I also reconnected the floppy power connectors back in and the floppy drives flashed at me and appeared to be searching for a disk. I was curious about not having the P8 connector plugged in and am scratching my head now about it booting to BASIC without it being plugged in.