I haven't tried that, seems unsafe.
No more unsafe than all the sticks installed. You are mixing RAM, that is whats happening. Brandnames don't matter a hoot. Corsair, G.skill, Patriot etc, they don't make RAM. They package it. And whatever ICs they happen to have on hand is what you get. With some binning, of course. Binning = testing capabilities like speed, voltages, timings etc. G.Skill seems to do a very rigorous binning process compared to others.
All that said, there is NO guarantee your sticks will work together UNLESS you bought them ALL in the same kit, be that 2 or 4 sticks.
Theres a workaround, which is manual tuning. You set the speed, the voltage and most of the timings.
Does your mobo list the RAM timings in BIOS? You could check what your mobo sets as Auto-timings for the new and the old RAM (1 stick installed at a time), compare, and go with the highest timing values (the loosest) for all your RAM, see if that works.