Can photorelays be used by series connections or parallel connections? If yes, what points require attention?

In principle, photorelays can be used by series connections and by parallel connections. However, there are important points that require attention. For series connections, voltage sharing between devices (distribution, balance) is important. Especially, within the transition time when the state shifts from OFF to ON or ON to OFF, in time differences that occur between devices connected in series, voltages must be balanced so that a voltage exceeding the value for OFF-state output terminal voltage V_OFF is not applied.
For parallel connections, current sharing between devices (distribution, balance) is important. Especially in the ON state, currents must be balanced so that ON-state currents flowing between devices connected in parallel do not become unbalanced or not exceed the specified value for ON-state current I_ON.