I’m using Comodo Internet Security ( Free Version ) 3.13.121240.574
and I’m also running Mailwasher ( Free ).
My question is should I be running Microsoft Security Essentials ?
Does it interfer with Comodo Internet Security ?
If CIS anti virus is enabled you should not install MSE or any other AV.
Never have two AV or two FW running together. At the best it will slow the system down; at the worst it will bring everything down.
Check the following for configuration of CIS https://forums.comodo.com/feedbackcommentsannouncementsnews_cis/how_to_achieve_max_security_with_zero_alerts-t44371.0.html
Don’t forget to turn Windows FW off.
Thank You Jose 
Just a clarification here, you should never run two software firewalls together. There is nothing wrong with having a hardware and software firewall running.