Can I mix RAM brands like Corsair and G.Skill?

Regarding the G.Skill: Yes, mixing RAM brands works more often than not, but it’s not guaranteed. Two sticks from different brands with the same specs (DDR4 vs. DDR5, speed, timings, voltage) will probably work, but the real issue is the underlying memory chips from different manufacturers. If they don’t match perfectly, you risk crashes or boot failures. Try them and run MemTest86 — if it passes, great; if not, buy a matched kit.