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Different RAM stics on the same Motherboard?

I was doing some research about installing RAM on the motherboard. I have Acer Predator G3620 motherboard, and I have a 2x 4GB UNB PC3-10600 CL9 DDR3 on it.

Now, I had this older computer that I want to use RAM sticks and combine with this Predator one because I had two more slots left on the motherboard. The following RAM sticks are:

I haven’t put them and tested it because I am afraid that something will happen.

I also ask because I have read somewhere that the order of inserting RAM sticks matters. I have different brands, same speed, all DDR3, but one of them is 2GB while others are 4.

Can someone help me regarding this? Thanks