I agree with the feedback from matteoguglielmi. I would also use 4 identical module sizes of the same manufacturer & type (in this case DDR3L-RAM, SODIMM 204).

Yes,

but you're better off with 32 GB of ram (4 new dimms) for two reasons:

1) all memory modules are identical which won't impact performance

2) DDR3-RAM runs at 1.5 Volt *only* while DDR3L-RAM runs at both 1.35 and 1.5 Volt

While in principle you should be able to mix DDR3 and DDR3L modules, I just wouldn't
because of 1) and 2).