I can confirm that the Acer Aspire E5-573G-56RG DVD slot can indeed take a caddy and SSD card. I installed the 9.5mm caddy from here
http/www.amazon.com/gp/product/B010CRVKUO?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00
and and a sata 3 SSD drive - I used this one http
/www.amazon.com/gp/product/B00M8ABEIM?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00.
Process is supper simple.
1) Remove the one screw that locks the DVD drive in. Its located about 5" in from the front of the middle of the DVD bay. Once it is loosened you can pull the blank caddy out. One you do unclip the trim and unscrew the metal piece at the back.
2) Clip the trim on the front of the new caddy. Screw the metal thingy on to the back of the new caddy. Insert the SSD in the caddy and tighten up the screws. Insert caddy holding SSD into the laptop and tighten locking screw.
3) Power on laptop. Boot to windows 10 and use your preferred partition tool to partition your new drive if you want to. Alternatively if you are installing windows then windows will do it automatically.
4) Assuming you want to take full advantage of your SSD drive you will want to either transfer your existing windows install to the new drive or perform a fresh install. I went for the later. The process was very simple. Attach USB DVD. Alter boot order in BIOS and boot from DVD. Select install windows 10 to your new SSD. Windows will partition, format and install windows. Very painless. You will have to manually install the NVIDIA drivers but other than that everything was included in the install.
5) Laptop flys with windows and apps on SSD and ACER bloatware removed. Enjoy.