the manuals at HP.com cover all that, RTM?I have 3 SSD in my Dell but dell is best.2015 PC yearbest i can tell (hp , has horrid indexing)this is your page
https://support.hp.com/us-en/product/hp-17.3-inch-laptop-pc-17-cp0000/2100371514page 9 (service manual)Up to 16 GB of DDR4-3200, DDR4-2666, or DDR4-2400 dual-channel support 16 GB (8 × 2)means 2 sticks of 8max. RTM manual any time you want. no lack of facts there.now the space issue HDD/SSD ? (as a 2nd drive)page 2 states sold with 2 drive option new, HDD + SSD1 TB, 5400 rpm hard drive + 512 GB, PCIe, solid-state drive but it does show M.2 drive ,so SSD is M.2 car/drive.M.2 2280 solid-state drives (NGFF) up to 1TB possiblepage 37 see, M.2 slot ,for a SSD m.2that is it , so RTM and page 9 here
http://h10032.www1.hp.com/ctg/Manual/c07605281.pdfssd and hdd both shown clearlyon mine it has DVD drive pulled and hdd caddy with SSD drivemaking mine 3 x SSD , but you have no dvd drive to do that trick but does now have m.2 slot for any SSD drive.good luck with all that.the service manual at HP.com is clear. and there24/7/365