Yes it is reliable BUT...at least you need a reliable hardware design to start, then everything will follow. The internal osc is never reliable for asynchronous serial communication anyways.

you could still share the OSC/Clockout of 1 PIC and connect all other pic to it... everybody will have the same the same reference so it could work. Never tried it, but in theory it have to work.

The Synchronous communication will work. You can use SHIFTOUT to send the data.. but you'll need to build your own routine to receive and decode it... or use the USART in synchronous mode.