Different RAM voltage

The boards will not provide 2 different voltages for 2 different dimm modules.
Low voltage dimms are designed to use less power, work really well doing so. From an overclockers perspective , they would hit it with 1.5v without flinching. I am not reccomending using things out of spec, just indicating it probably is not a big issue in this situation.

You still have the problem of these not being designed to work together, and for it being "best" to have a set of ram modules that are exactally the same, came as a set, are from the same batch and everything. so you are not following that recommendation first, it could be very important, or not.

Assuming you have 1 stick now? adding in a second stick to the proper location would get the rams to be working in dual channel mode. Dual channel is going to be even harder to get mixed up combinations working correctly.

There are no Facts in this post, just thoughts. If you have not purchaced yet, then buy matched set rams that are on the compatability list. If you want to guess and hope, then go ahead, and make sure you test the ram for like 24 hours , or 2 nights or something with memtest, before believeing everything was fine.
The minor and lesser seen problems that could possibly occur from this kind of thing, are a bigger problem then doing things 100% according to the book.