Since you need mutiple masters (where any device can initiate a conversation), I2C is possible, but is a terrible mess.

I recommend that you use RS-232 in its open-collector variant. It is much easier to run multi-master, and PBP already has extensive support. It greatly simplifies things especially if the PIC you choose has a hardware USART.

I run 38.4 Kbaud on my little PIC network without any extra hardware support. If you have the ability to add one open-collector gate to every device, you can run up to at least 115Kbaud. I communicate between an 18F8723 and several 18F2221's. The 18F2221's are running on their internal oscillators. The network has been very reliable across wide temperature ranges.