The 10 cameras together need 10*8 = 80mA on the opto coupler inputs, what's too much for a single Arduino pin, but not for any discrete transistor/FET. Opto inputs are LED, i.e. every input needs a current limiting resistor (~390 Ohm at 5V). Then multiple inputs (1-5) in parallel can be driven by one Arduino pin, more pins for more inputs - the simplest solution. Or you add a transistor or logic driver gate(s) to one output pin, to drive all opto inputs in parallel.
For testing I'd use the simple solution, to trigger e.g. 3 cameras from one Arduino pin. It's up to you to find out how precise the timing of the cameras is with this approach. Taking pictures of a moving object (pendulum, rotating wheel...) should reveal possibly different delays between multiple cameras, which cannot be compensated by any electronic.