You don't need the same RAM, any RAM fit the correct spec can do.
For stock 5,1. It shipped with DDR3 UDIMM ECC RAM. As long as you buy DDR3 UDIMM (desktop RAM of course, not notebook RAM). All your RAM should able to work properly. ECC is not a must, but highly recommended, and ECC RAM is actually cheaper now.
Just replace all of them with cheap Samsung RDIMM DDR3 ECC memory pulled from servers. It works great. About $20 for an 8GB stick or $50 for 16GB sticks. Get PC3L-10600R(DDR3L-1333) speed or faster. The memory for the Mac Pro 6,1 works fine too. Get dual rank memory denoted as 2RX4.