XPS 8300, different ram sticks and speed together?

It's never a great idea to mix different speed RAM. Dells are picky about RAM to begin with. But besides that the 8300 specs call for DDR3 1333 (though DDR3L 1600 would work per crucial) and a maximum total of 16 GB. That would be 4 slots of either 1, 2, or 4 GB with totals of 3, 4, 6, 8, 12, or 16 GB. Not 24 GB.

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/xps-8300

Dell doesn't test every possible RAM config, but that doesn't mean something they didn't test won't work. But you can't complain if/when you start getting BSODs with an untested RAM config.

And keep in mind that all installed RAM runs at the speed of the slowest RAM that's installed. So in your case, it's all running at 1600 MHz.

BTW: I think you meant Corsair 8GB DDR3-1866. 

It doesn't matter, as long as the 2 matching 8-GB modules are in slots with the same color clips.

And you can keep both the 2x4 GB and 2x8 GB (= 24 GB) if that's working without BSODs, as long as each matched pair is in slots with the same color clips.