RAM x MOBO Questions

Solution
You can try, adding DRAM, even the same exact model, is a 50/50 crapshoot, no way of telling if they'll play - if not can face return fees, restocking fees etc. Best bet is get a 2x8GB set of in a single package
1. If you are building new ... 2 x 8GB is a batter choice than 4 x 4 GB .. less strain on memory controller and going to 4 may reduce OCs.

2. There's a picture in your manual as to what sticks go where with any combination of sticks.

3. If you buy 4 x 4GB, the manufacturer guarantees that they will work at rated speed. If you buy 2 packs of 2 x 4Gb, you have no such guarantee. Might work, might not... may have to reduce timings, reduce speed or bump voltage to make it work. Generally tho, it will work at the slower DDR speeds... the higher the speed rating, the less likely they are to all "play well together".