Late 2009 27" iMac to Current rMBP as display?

[Kabir8]

I have searched and got some conflicting information, but here's the situation:

My late 2009 iMac is great, but slow.
My brand new macbook pro retina (fully loaded 15") is blazing fast, but I am used to working on a large display when editing.

I just ordered the mini displayport to mini displayport cable, and just wanted to know the protocol to make it work most effectively or if there is an issue if I have 10.7.5 on the iMac vs Mavericks on the rMBP.
Just want to know if it compatibility and/or latency will be an issue?

Thank you!

 

[mcdisease]

My wife has a similar set up. We ordered this cable and it worked well:
http://www.amazon.com/gp/product/B002YHYTI6/ref=oh_details_o01_s00_i01?ie=UTF8&psc=1

The difference we have is that her MacBook is for work so she'd still like to be able to use her iMac most of the time. While working from home she wants to connect her MacBook to her iMac and use it as an external monitor, but the keyboard and mouse are still connected to the iMac. I was able to turn off Bluetooth and connect her keyboard and mouse but now will have to hard reset the iMac to use it later. Not a good solution going forward.

Anyone have ideas for sharing her iMac's keyboard and mouse with her MacBook?