Battle Crystal Security + WD vs. Bitdefender Free

@Kardo Kristal recommends using Crystal Security as the second layer of defense. It is compatible with any antivirus and really lightweight. As @Spawn says, the three can work together providing you with an excellent defense
three can work together but WD will act as on-demand scanner if you install Bitdefender.
 
I just use Bitdefender Free. For some reason I don't trust these second layered protection softwares, if I wanted more protection I would install Comodo Firewall.