Same make/model (exact match) is the best bet.
If you purchase multiple speeds, ALL of the RAM will run at the slower speed.
Theoretically, the brand of the RAM doesn't matter if all the settings are the same (not just frequency - but the timing settings).
If you get RAM that has different timing settings - be prepared for problems (if it works at all).
Even with the almost exact matches, it makes me nervous. I typically buy the RAM as a single purchase just for piece of mind, and I save the old ones for upgrades or a new build later. Of course, over time, some of the RAM (remember those 1MB modules - LOL) have either been thrown out or recycled....