Yeah the 1866 RAM should work fine for those motherboards. Also you can use different sticks of ram on your mobo but I wouldn't recommend it.
1. The timings of the RAM will be different from eachover.
2. The speed of the Ballistix Tactical Tracer will be dropped down from 1866 to 1600Mhz automatically by your system so it would be a waste of money. Best thing to do is buy a full kit of the new ram you want.
Hope I helped