how to Mixing 2Rx4 and 2Rx8 ddr3 for t620

my dell poweredge t620 has 32g memory with samsung 4 x 8gb ( 8GB 2Rx4 PC3L-10600R) in slot (a1, a2, b1, b2)

I hope to update the memory to 64G,and bought kingstone 4x8gb ( 8GB 2Rx8 PC3L-12800R).

i've tried plug the kingstone in (a3,a4,b3,b4), and the bios detected 64gb ok，but the system displayed 64gb with only 32gb aviable.

i)  anyone tell me how to mix them ? and the bios setting should be changed from optimized to addvance ecc OR not?

ii) the manual shows that  (8gb 2rx4)  can be used for 64gb instead of  (8gb 2rx8) , if i got 8x8gb ( 8GB 2Rx8 PC3L-12800R)  for memory update is ok?