Can I mix single-rank and dual-rank RAM?

Probably yes. Matching speed, timings, and voltage matters more than rank. A dual-rank stick added to a single-rank stick of the same specs will almost certainly work. The main risk is on older boards or with four sticks, where dual-rank can force lower speeds. For a modern two-stick setup, you likely won’t notice a difference. Run MemTest86 after installation to be sure.