Q: Late 2013 27 inch iMac Upgrading RAM

No, my iMac has 24GB and runs perfectly, it has for the past 2  years.

What the sequence of modules installation? 4-4-8-8? 8-8-4-4? Or 8-4-8-4, 4-8-4-8? Thank you.

You are making this WAY too complex, simply install the RAM into the empty slots and get on with  it.

i've read something about the performance reduce after the incorrect installation of the two-channel memory. and in some scenarios mac does not boot up.

Read the user manual on RAM installation that came w/your iMac and/or read the KB Article iMac: How to remove or install memory

I'Ve read this article many times and there is no one word about the installation of different memory size RAM modules.

You can install the ram in any of the 2 empty slots. It can be just one or two doesn't have to be in pairs. Go for the max if you can.

It is not necessary to match pairs with memory, but it will run better if you do. On the store website they were only configurable from 8 GB to 16 GB or 32 GB.

iMac (27-inch, Late 2013) - Technical Specifications