There are two widths of chain: 3/32" and 1/8". 3/32" is what derailleur setups use, period. Some track setups use it as well, but 1/8" is the traditional. It's also the standard BMX chain width. On top of that, a single speed specific chain will not have plates designed to work with shifting like a 3/32" derailleur-intended chain. It's that last part that he's probably warning you about. It's not usually a problem but there is some chance that putting a multispeed chain on there may allow it to "shift" off into space.

Without knowing any better, you can always fit a 1/8" on though if you have 3/32" components it may be noisier.