regarding the mixing of ram: Don’t. Your wallet may hate you but it’s worth shelling out for same make and model of RAM. Exact match only. getting 1 pack of 4 sticks is still as much a gamble as getting 2 packs of 2. You can often save yourself $10-20 by getting 2 packs of 2.
Simply put: your computer will take the slowest stick and set that across all the modules. not to mention a myriad of other issues that come associated with mixed RAM modules. Some people can do it and not have an issue. Others do it and they brick their motherboard. These are things that can happen to anyone, so don’t worry. As long as your handle your computer parts with care and be sure your hands are dry (i had to buy a new mobo as a result of a minisule drop of sweat on my finger)
Regarding 2 slots or 4: This will always be an ongoing debate regarding performance of 2 slots over 4. my belief is you can achieve faster clock speeds with dual slot over quad, spoken from experience. I used to be able to run 3200mhz at 2x8gb. I have 4x8gb (all matching modules) and can only run 2966mhz before my system decides crashing is a regular process. Again, this is highly debated due to the amount of configurations are on the market.
Your supplier is just trying to get your money. While valid, i feel like this is just to sucker you into spending another $200 on RAM you don’t need. This is an issue with IT because suppliers, resellers and repair shops will prey on the ignorant. Not speaking to you specifically but i’ve worked in the industry long enough to see.