You can not mix memory of different size on PowerEdge R6515. You can refer below link for guidelines. 

https://www.dell.com/support/manuals/en-us/poweredge-r6515/per6515_ism_pub/general-memory-module-installation-guidelines?guid=guid-80b1c1ad-14b7-4dd6-b122-abb0c82bd3e8&lang=en-us 

just another tip, if you end up getting say 2 x more 64GB dimms make sure they are the same speed or else they will all run at the lowest speed of whatever kind you have