Would it be wise to turn off Windows Firewall, as CIS Complete already has one built in? My only other question is since Windows Defender is pretty much built in, can/should it be disabled or do they play nicely together?
I have disabled windows firewall and windows defender
(my OS is Windows 7 Ultimate x64).
I think that CIS is quite enough 
You should never run two firewalls at the same time. And even if you did, yes, there really is no reason, as CIS covers everything. I have my Windows Defender disabled, but I used to use it with CIS. I never experienced any problems with it conflicting with anything else.