Multiple Antivirus Software

McAfee Security Scan Plus is not a pro-active virus scanner, in fact, it's not a virus scanner at al. It merely checks if you have a firewall running, if your virus scanner is up to date and checks several other factors that may lead to your computer being compromised.

So yes, it is safe to keep McAfee Security Scan Plus installed, but if you don't need it, it's safe to uninstall as well.

Correct, standard practice is to have only one AV application installed at a time, as they are quite capable of perceiving each other as threats and locking up your Internet connection. And yes, you should go ahead and delete it since you didn't want it in the first place!