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AFAIK it doesn't *have* to be in equal capacity pairs, however not doing so will impact memory performance slightly.

Whether you'd ever notice that performance impact is a whole other matter.
 
My late 2012 Mini came with 4gb installed (2 + 2).

After about a year-and-a-half, I bought an 8gb RAM module. It was very easy to remove the top 4gb module and replace it with the 8gb.

This gives me 10gb of installed RAM.
I realize the different RAM sizes -may- result in slightly reduced speeds, but it's nothing that I can actually "perceive".

It runs as fast as it did before...
 
I've got a late 2012 Corei7 Mini, it came with 4gb, I installed Parallels and it ran like a dog, so I went out to buy 8gb, on the way the shop sold the 2x4gb, in the end I bought 1 stick 8gb. I've only got the 8gb in there at the moment, but I'm going to stick one of the 2gb in there as well, what can go wrong.
 
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