I used to pay subbies under the SC60 scheme but since I did it the rules have changed. When I did it you did not need an invoice from the subbie you just kept a record of payments on the green card and reported the figures at the year end. Just give the tax office a call and ask them what the current situation is; you are quite correct though to make the deductions unless they are officially self employed and therefore responsible for their own tax.
You will find that the tax office will be very helpful.