Can I mix different RAM sticks safely?

Usually, yes. You can mix a 16GB stick with another 16GB stick to get 32GB, and it will work in most cases. Matched sets are better because they’re tested together for stability, but the difference is rarely noticeable for everyday use. The risk is that mismatched sticks may cause crashes or force your system to run at slower speeds, so try the cheap route first and return if unstable.