Mixing RAM is problematic.
Generally you can install faster RAM into a system along with slower modules but the system will only perform at the slower rate.
You must refer to the motherboard's documentation (User Guide/Manual) to determine what specific RAM modules are supported and in what slotted configurations.
Most motherboard User Guides will refer you back to the motherboard's manufacturer's website for more information and details. E.g., the QVL (Qualified Vendors List).
Another source of information are the various RAM manufacturers: PNY, Kingston, Crucial, Samsung....etc..
You visit their websites and enter basic information about your computer and the website will provide a listing of the RAM supported by your computer/motherboard. Not always 100% accurate but a consensus on the specs can be helpful.
Knowing your motherboard's make, model, and revision is the starting point.
Details matter so read all the notes and fine print both in the manual(s) and online.