I'm not the greatest person with PCs, my mobo doesn't support SLI, but it has PCI Express 2.0 x16, will a SLI card still work on it, just not running in SLI mode?
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).
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).