You're changing to a new single phase DB (distribution board)..

A DB has a 2p (2 pole or double pole) isolator => 2p rcd(residual current device), rccb or rcbo => Neutral bar + SP(single pole) mcb(miniature circuit breaker) => outgoing wiring to lights & socket load..

Your picture shows a 2 pole isolator for the neutral and phase cable i.e single phase ..

A common rating is 40A DP to protect 16mm2 pvc cable, use the 63A only if you are going to use 25mm2 (or larger) size cable..

Please refer to Suruhanjaya Tenaga's Guidelines for Electrical Wiring in Residential Building 2008 (Guidelines-Electricity) if you require more details..
Yes..
No.. 
Yes..  The reason is due to its nature of operation.. The current flowing through the phase cables MUST return to the neutral cable..

Any leakage current will be detected by the residual current device and trip the supply..

Thus it is 2/3/4 pole..  a source(single or 3phase) and the return current path (Neutral)..

The 2 pole is for neutral + live. Not for normal house DB. It is for like near to the aircon isolator box. Or near to the motor isolator box. Mostly factory use.

You can separate out by take out the joint black trigger. But some of them joint up internal or a bar across.

RCD and RCCB require 2 pole to function. Live and neutral. 4 pole is for 3phase. Both available for residential use. Depend you using single phase or 3 phase.

Return back.

It is no use for you to put 2pole in the DB. Cause the neutral will be join in common.

If you insist use, the guy will just join the live to 1pole in the 2pole MCB. Left another pole empty. It still function normal. But eat up your DB space.