iMac 27 3.2 GHz core i5 [late 2013] memory upgrade I'm thinking to upgrade memory on my iMac 27 3.2 GHz core i5 [late 2013]. It has currently built-in 8GB [4GB x 2] RAM. My questions are:

1] Can I upgrade RAM by simply adding 16GB [8GB x 2] in two empty ram slots? Or is it necessary to have 4GB x 2? Does it have to be 4GB... won't work with 8GB module? Is it that all modules have be with same capacity? Can I make it [4GB x 2 - built-in] + 8GB x 2 - to be added] 24 GB? 

2] Does memory module designed for i7 work in i5?
Yes, you can mix and match memory modules with different capacities with no problems. My own late 2009 iMac is currently running with two of the 2GB modules it came with and two 8GB modules, for a total of 20GB. However, for the best performance you should "pair" the modules, and make sure you have two identical modules in adjacent slots.
There's no such thing as memory designed for an i7 or i5. What you need are 204-pin PC3-12800 (1600 MHz) DDR3 SO-DIMM modules, which can be used with either an i5 or i7 CPU.