Firstly, put varistors across the solenoids and motors. Even with optoisolation, I've found unprotected inductive devices cause enough interference to go across the optoisolators.

As for a split plane, probably not necessary if you are real careful as to where the current is flowing across the board.

I recently did a project switching 20A DC motors with a microprocessor controlling it. Common ground plane on the pcb. With my prototype I found the optoisolated usb->rs485 adapter I was using would 'lock up'. I did some investigation and found the motors were generating enough interference to cause it - funnily enough the microprocessor on the board was not affected! A varistor across the motors solved the problem.