Is using mid 2012 MacBook Pro RAM in mid 2011 iMac 27" an option?
While I am not a fan of of mixing and matching memory, Crucial, a manufacturer of quality memory components, has this on their website:


  Memory is designed to be backward-compatible, so generally speaking,
  you can safely add faster memory to a computer that was designed to
  run slower memory. However, your system will operate at the speed of
  the slowest memory module.


So, technically speaking, the answer to both your questions is, yes it will work and no it won't make a difference as you can mix and match the different speeds.

They also go on to say...


  Keep in mind, that the right memory for your computer is the kind of
  memory it was designed to take. Check your system manual or look up
  your system in the Crucial Memory Advisor or System Scanner tools to
  find the memory guaranteed to be 100 percent compatible or your money
  back!


Well, the "system scanner tool" is for PC's but the rest of the statement is absolutely true.  The last part about "money back" is why I only buy from reputable manufacturers like Crucial (PNY and  Kingston are excellent choices as well).

That said, according to Everymac.com, the maximum supported memory configuration is 32GB1.  Personally, I would (at minimum) sell the  existing memory that you have (4GB modules are going for around $20USD on eBay) and instead buy a  16GB kit so you can max out your memory in your iMac either today or at a later date.

My personal iMac runs with 32GB of RAM which allows me to virtualize a Windows 10 machine, and two FreeBSD servers (one for development/testing and the other for a Plex media server).  I am a firm believer in the old axiom "it't better to have and not need than to need and not have."



1 Everymac.com:  Officially, this model supports four 4 GB modules -- one in each slot for a maximum of 16 GB of memory. However, third-parties have discovered that it actually will support 32 GB of memory using four 8 GB modules.