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