Upgrade RAM with two different size.

Doing so will cause your system to operate in single-channel mode, cutting memory bandwidth in half.  

Mixing brands/types of RAM is iffy. We see a lot of memory problems over the years because of mixed memory (and this is not just a Dell issue - any brand).  If you want to try it, only buy it where you can easily return it if it doesn't work.  

This means I've to add 8 GB RAM, not 16 GB.

Yes, modern intel and AMD motherboard support flex mode when you mix memory size.

1. check what Dell offers for the model, in this case Parts & Upgrades for your Inspiron 13 5000 (5378) 2-in-1 | Dell Canada
2. Look for same voltage and same or > MHz among Silicon Power, Timetec, Kingston, ADATA and Crucial brands of RAM.
3. I have Amazon Prime, so easy returns, but I've yet to go wrong when buying RAM.