Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
In answer to your first question:
I think you would be well-advised to use shielded cable! it can prevent crossover of signals, and in an alarm system, you do not want anything of the sort.
As for the second question:
I'm not sure of the wiring as to strobe-to-strobe. As to how much you can add on the circuit, that depends upon the loading capability of the output circuit, and also whether they are powered independently of the circuit or (like a USB device often is) are powered by the circuit. Any which way, they must have a common Ground for sure.
Since my answer to your second question is not complete and certain in all details, I apologize and have Opted Out, opening the question for other Experts as to the second qeustion's details, about wiring and number-of-possible-devices-in-circuit for an FS-250 alarm panel.