I also have a 2010 Mini, can I put the 2012 memory in the 2010?
Yes.
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Not really. 1600MHz RAM in principle
will work in a 2010 Mini. He has two options:
1- keep one of the original sticks (in the base model, it was a 1GB one) then install another one with 8GB, totalizing 9GB. The important thing to note is that you
must keep at least one 1066MHz DDR3 stick. The other one can be a 1333MHz or 1600MHz one.
2- you can install two 8GB 1600MHz sticks on a 2010 Mini, but you have to convert at least one stick to be recognized as a 1066MHz one. This can be made with an app called Thaiphoon Burner (it costs $10, Windows-only). After installing this app on a Windows machine (or in the target Mac, if it has a Windows installation via bootcamp) you can rewrite the description label of a stick making it to be recognized as a 1066MHz one.
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Can I put RAM from 2012 mini in Late 2009 mini?
Going to upgrade my new 2012 mini to 16GB, wanna put the 4GB in my extra mini, the late 2009 model. I realize the RAM won't run at full speed but it has the same number of pins. Can I do this or not?
If your Mini has a Geforce 9400m GPU, the maximum RAM supported is 8GB. There are not very accurate reports that say it supports up to 12GB, but I never tested. Also, I don't know if it will support 1600MHz (from the 2012 Mini) RAM downrating it properly to 1066MHz.
The 2010 Mini needs rewriting of ram descriptors on faster (>1066MHz) sticks, but I heard that 2009 Minis and 2009 13" Macbooks don't require this, although they're limited to 8GB (4GB+4GB sticks).
