Can I use slower or fastest Memory in my MacBook Pro mid 2012? Different speeds? Different sizes? I’d like to know if there’re any side effects to these memory configurations for MacBook Pro mid 2012 (I guess this can be extended to any Macs with removable memory).

Doubt 1:  any side effects if I use slower or faster memory?

Doubt 2: should I use different sized modules, such as a 4GB in one slot and a 2GB module in the other?.

Doubt 3: can I mix the speeds (such as 1 memory slower than the other).

Doubt 4: is dual channel used in the 2012 Macs?  (I can’t see this stated in support.apple.com/specs)
Only two MBPs match your description, and they both require:

204-pin PC3-12800 (1600 MHz) DDR3 SO-DIMM.

Either MBP 13" &amp; 15" can take 16 GB (actual) but are equipped by Apple with 8 GB. 

Simple answers:


Use the RAM speed the MBP was designed for (in this case 1600 MHz.)
You can use differing sizes, but performance will suffer as interleaving (dual-channel) will be disabled.
Do not mix speeds (see #1.)