Piyal,
well the way I see it, if you have not flown before then it is probably good to start on main fleet, see lots of destinations and you wouldn't know any different with the turnarounds and 'bad' flights like people who have flown before. You could apply for A380 after 6-12 months (or maybe sooner if they are desperate for crew!)
But for those who have flown before it seems lots of them get put to A380, maybe they like experienced crew there. If you don't really feel one way or the other then don't worry about it, but if you really wanted A380 I suppose it wouldn't hurt to say so.
Since you already declined them though my guess is main fleet. You can try applying later perhaps.
Yeah the EK website is a good place to find the answers for your book, its how I filled mine out
