Why is it okay to use multiple anti malware, but only one type of antivirus

[kapura.ftw]

I am trying to understand the different types of protection that a computer needs.
This is what I have learned:
-You cannot have multiple anti-virus software on your computer, because they will interfere with each other
-You need to stack up other anti-malware software, the more the better
-You can not have two firewalls on one computer, but you can have a firewall on a computer, then a firewall on the server that the computer is a part of.

But if anti-virus is a type of malware, why is it that you can stack the other types- but not virus protection?
How does the firewall on the PC that goes through the servers firewall not conflict?

Thanks for any information, I'm just struggling with how things ACTUALLY work. Its fine that this is the info that my teacher/my book want me to know, but I need to know WHY as well :)

[CimmerianX]

Antivirus programs are usually 'Active'. This means that the program starts with the OS, is always running in the background, constantly checking files, webpages, downloads, etc.... it is always running. If you have 2 of these running at the same time, they start to step on each other every time they both want to check the file you just opened.

Antialware (i.e. malware bytes) is usually passive. You load up the antimalware run your scan, then you are out. Some, like spybot, can manipulate certain files (i.e. hosts file) to help protect you, but again you are in, run it, then you are out. Some malware apps do have a feature to always run in the background, but it's not a requirement like AV.

Firewalls enable/disable certain traffic. You only need 1 firewall. If you have 2 you double the complexity, but get no additional return.