EH...it's next year only. Keep quiet about it for now. Mention to them before you sign offer letter (if it falls within the probation period). Otherwise apply leave like a normal person lah.

Depends on the company and manager.

If company enough people with no heavy projects in hand, then OK
If manager know how to manage then OK
If not many staff is taking leave at the same time, then OK

If you start of as a management trainee, usually OK
If you are in sales, on leave means missing sales target then miss out monthly comiission.

When apply, use your annual leave (AL) first
Do not need to volunteer to use unpaid leave.
You only need to apply AL on work days.
That means if Sunday not a work day then Tue to Sat = 5 days
Sunday no need to apply. Only appply 5 days.

Usually apply require 2 weeks notice or 1 month notice.
Do so then no problem.