Buying the correct RAM Brand and Upgrading RAM in correct way

Hellooo

I am recently doing upgrades in my late2015 27 iMac 3,2 GHz (Skylake). Now I’ve decided to go for RAM. So by default, I’ve got 8 GB RAM(Two 4GB DDR3 1867 MHz PC3-1490 0 Unbuffered Nonparity 204-pin) placed originally as:

BANK 0/DIMM 0 4GB DDR3 1867MHz

BANK 0/DIMM 1 EMPTY

BANK 1/DIMM0 4GB DDR3 1867 MHz

BANK 1/DIMM 1 EMPTY

As seen above I have two empty RAM slots: I want to populate them with two 8GB RAM as to have total of 24 GB RAM. It is pretty straightforward to install RAM however:

1) Which combination of BANK’s and DIMM’s should I go for when installing RAM? There are different opinions on this, creating confusion…

2) Does having different sizes of RAM(in this case two 4GB and two 8GB) prove to be unhealthy in terms of general system performance compare to having same sizes of RAM in 4 slot (ex. four 4GB or four 8GB). Is there any issue here ?

3) There are some (not few!) negative user feedbacks about using 3rd party RAM in Late 2015 iMac; generally stating crashes after 12 to 24 months of usage. And all these crashes seem to get better after re-installing original RAM that came with iMAc. So which brand should I go for in order to avoid these setbacks as much as possible ?

Thank you in advance