You can run 2 different K bulbs, but you will be able to tell a huge difference in the K. especially with only 2 bulbs, one on one side and one on the other. I run 2 different K bulbs, but I have 3 bulbs and the center one is the one that is a different k. And you can the difference between the K's
If you go with MH I would supplement them with VHO (all Actinic). On a 75 you can run just two bulbs, but you will not be saving "much" more money than just buying the set that runs 4 bulbs, other than cost of the extra bulbs.
On the timers "most" people turn them on and off at the same time. Now if you add the VHO you will want to run them at least an hour or so before and after the MH's go off.