Can I break a single speed chain with kmc universal link It maybe necessary for me to remove the chain for a deep clean. I dont think it's ever been cleared by it's previous owner. So just to be sure I can use one of these to rejoin the chain: < wanted to make sure just in case I screw up a perfectly working chain. It is a kmc chain I believe.

It depends on the width of the chain. Single speed chains come in 1/8" and 3/32" widths, so you need to know whats on the bike to use an appropriate master link.

You should also check the chain, chainring and cog for wear. If the chain is old and poorly maintained it may be time for a new one (and possibly replacement of other drivetrain parts).