Thats not true at all dark.
I think you're misunderstanding how it would work.
You would still end up with 12gb of ram it would just end up running at the slowest ram speed.
The reason sometimes this don't work and cause other issues is ram timings and things of that nature, personally I would try both if everything worked fine great if not I would simply just stick with the kingston obviously.
Would the 12GB at a slow speed be better than the 4GB of ram at the normal speed? For complicated reasons, putting just the kingston ram in isn't an option at the moment.