A video card with SLi support doesn't need to be running in pair with another one so the answer to your question would be that you can indeed run a single SLi ready video card in the appropriated slot.
SLI support is literal. Support. You can run it stand alone, but have the option to add additional cards via SLI. You're fine, compatibility-wise, and it will work 100% correctly (barring any other unforeseen issues).