yes well with your SSHD, you might need to use Diskpart to clean it. your old system was probably BIOS MBR, and your G551 is UEFI GPT So you'll need to either Clean and then convert to GPT. Just clean should allow your system to boot and clean install or Clone your OS to it. Any 2.5" SSD should work, the best seems to be the Samsung 850 Pro or Evo. Coming from the manufacture in clean condition to allow a UEFI Install. See this thread on
how to install correctly UEFI . It appears that you don't have an M.2 slot so yes the HD can be switched out for a 2.5" SSD The EVO are good with 3 year warranty, the Pro's are 5 year warranty both come in 128, 256, 500, 1 TB configurations.
https://rog.asus.com/15842014/hands-on/g551-upgrade-guide/
G752VY-DH72 Win 10 Pro
512 GB M.2 Samsung 960 Pro
1 TB Samsung 850 pro 2.5 format
980m GTX 4 GB
32GB DDR 4 Standard RAM
Z97 PRO WiFi I7 4790K
Windows 10 Pro
Z97 -A
Windows 10 Pro