Mike, regarding connecting the grounds together, lemme make sure I understand you. Are you saying that you'd connect grounds on each side of the isolator individually, but then not connect the two sides together?
If I used solid state relays, it appears that they typically have a + and - connection for each side. Assuming there is an opto-isolator built in to the SSR, I don't see how you'd separate the ground on each side of it.
Would it be correct to connect them to a GPIO pin and ground (respectively) on the control side, and then connect the switched side between 120V AC line and the device I'm controlling? I'm looking at a datasheet for an SSR from opto22 and it looks like it's really that simple (current limiting resistor built in for the LED, built-in snubber) - or am I missing something?