Will adding a stick of RAM work if my laptop has mismatched memory sizes?

Yes, it’ll work. Flex mode handles mismatched RAM: one channel gets the full soldered 8GB plus the SODIMM’s 8GB, the other gets just the soldered 8GB. The system runs dual-channel for the first 16GB and single-channel for the rest. No stability issues — just match the DDR generation, speed, and voltage (usually DDR4 3200 or DDR5 4800). The performance difference between flex and full dual-channel is under 5%, and doubling your memory is a massive win for multitasking.