You need to apply for the SIA positions seperately. I have found them to be very busy and it wouldn't necessarily occur to them to consider you for Cargo if you have only applied for main-line. In fact it would almost definitely not happen
I have known them to have problems with e-mail. Particularly when responding to you. So I "couriered" my application. It took one day to get to Sing from UK and e-mail response within one day with an interview date.
Beware though, if you don't hear from them give them a ring. If the e-mail reply goes astray they might just think you're not interested.
Good luck. O'Neill No6