Iver,
To answer your Qs:
1. Lately all SIA Mainline cadets are trained for B777.. You can't choose which fleet to be posted in, it's based on whichever fleet needs the pilots.
2. Hard to tell the percentage.. although majority will go to SIA Mainline, but again, that depends on each companies' needs...
3. No one really knows how's each companies select their cadets, unless you're one of the interviewers... But yes, there's transfer between these companies, although it's not as easy as it seems...
4. Well, depends on your performance, seniority, luck and many other things.. But around 10 yrs is a good gauge...
JA