Finally, SingTel now support MicroSIM cards in their MultiSIM plans, however the price is 2x more expensive the normal SIM cards and from the description on the VAS page under “Others” tab, it was mentioned that then can be only 1 MicroSIM card in any combination of MultiSIM.
So if you got 4 SIM cards, only 1 of them can be MicroSIM and the other 3 is normal SIM. Perhaps it is due to the initial hight stock demand. But I believed that it will change eventually once the dust has settle.