Can I use slower or fastest Memory in my MacBook Pro mid 2012? Different speeds? Different sizes?

1. Faster rated memory will not run faster or may not work at all.
2. Slower rated memory will not run any faster than its rated speed.
3. You can use different module sizes provided the computer does not require matched pairs.
4. You can mix speeds but it is not recommended. If it does work the memory runs at the speed of the slowest memory.
5. No, dual channel memory is not used in that model.
6. The rule is simple: Only use memory rated the same as that stated in the computer's documentation or the same as what was installed at the factory.

from readers' experiences here, FASTER modules are flaky, crashing randomly a few times a week.

Mismatched sizes can be used. If this gets you beyond the 6GB or less performance Rut, the minuscule performance decrease from mismatched modules is completely overwhelmed by not having to use much Paging for ordinary tasks.

The actual limits are 8GB modules in each of the two slots for a total of 16GB, PROVIDED you obtain the exact correct spec RAM that runs in these MacBook Pros.

The best way to obtain the correct spec RAM is to deal with a Vendor who knows Macs, and can promise you the memory They recommend for your model Mac works in your Mac, or your money back.

Otherwise, you may end up owning some excellent memory that does not happen to work in your Mac, and is not returnable.