Mix n match RAM memory in iMac with (potentially) an empty slot remaining [duplicate]

I'm experienced with Windows, not so much with Mac. I've just been given a 27" iMac (it's a mid 2011 model) by someone who no longer needs it. It has 4 memory slots and presently two are occupied with 2Gb modules each (4Gb total RAM).

Another friend has offered me a 4Gb module and a 2Gb module. I've checked the specs and they are compatible with this model iMac.

Is it an option to put each of these in the 2 remaining slots, so that 3 slots have 2GB and 1 slot has 4Gb?

While this question: Is using mid 2012 MacBook Pro RAM in mid 2011 iMac 27" an option? is similar, it does not address the following question I have:

And if one of these is bad memory, can I only use one? By this I mean 2 slots have the original 2Gb modules, one slot has either the 2 or 4Gb module, and the last slot is empty.

There are multiple iMac models, but in general, look in the apple menu and select 'About this Mac', then (in the window that opens) click on the 'Memory' selection. This will show your memory arrangement and offer "Memory upgrade instructions" if the computer is internet connected.

If the memory arrangement recognizes all the memory you have installed, it's working (though matched pairs are often preferred).