Hello BA,
as for "How can we assign more than one PR Doc. type while generating PR through MRP?" -- see customizing of MRP Groups (SPRO: Materials Management -> Consumption-Based Planning -> MRP Groups -> Carry Out Overall Maintenance of MRP Groups), under "Scheduling/Document Type" you can define different PReq document types used for MRP Groups.
The MRP Group can be then either assigned to Material Master (in view "MRP 1"), or specified per Material Type (see the customizing task "Define MRP Group for Each Material Type".
Kind Regards
TomT