Can I upgrade a Mac Mini (late 2014) with 8g ram with another 8g?

No sorry, the 2014 Mac Mini's RAM is intergrated into the logic board and can not be upgraded.

see > https://support.apple.com/en-us/HT205041

There really is no need to add RAM, your computer has a built in system to makeup for the amount needed. Open Launchpad and Search Activity Monitor > Memory You will see at the bottom "Swap Used". If you run out of RAM the computer uses a part of your HD as a swap file, like Linux has. See my screenshot below.:

If your Mac still isn't working properly, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as shown here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

You are welcome. Is the cursor mouse hooked up via USB chord, Dongle or Bluetooth. If you are still using the original HD it could be slowing things down. Recommend an SSD or NVMe for speed. As to the third party software, I have no used for them and have no recommendations, sorry.

Recommend a USB mouse with chord. Mine is a Logitech M100.