Nameservers In VPS - Domain In Godaddy - Backup - How To Setup?

I just got a VPS plan for the first time where I want to host several new websites. In the past I've used individual accounts to host a couple of other websites and haven't paid much attention to nameserver setup, just configured at my registrar whatever I was given from the hosting service.

So, I got a brand new domain to act as the host of the rest of the domains I'm setting up: www.example.com.

In the VPS I have setup this one as the default domain for nameservers: ns1.example.com
ns2.example.com

Question 1:
- Can I setup ns1.example.com and ns2.example.com as the nameserververs for www.example.com ? i.e. nameserver is used for the same domain in which it is named. Even if this is possible, is it a good idea or would it be better to host it somewhere else? When I try to define those as the nameservers in Godaddy I get the error message Nameserver not registered. What am I missing?

Other DNS questions
Question 2:
- If I host my nameservers in my VPS, I will need a separate IP for each one (ns1, ns2...and so on). Can I still use those IP's to actually host websites, or do I need additional IP's to host the actual sites? Any value in keeping each IP dedicated to a nameserver?

For example:
IP#1 = used for ns1
IP#2 = used for ns2
IP#1 = used for website 1
IP#2 = used for website 2


Question 3:
- Are DNS backup services valuable if I host my own nameservers? Any suggestions on good quality providers? Price not an issue.

Question 4:
- If I used DNS backup services for example.com will that apply to ns1.example.com and ns2.example.com which I used for my VPS? and therefore will be applicable to other domains I host? Does this make sense? Or would I need a separate backup for each domain I host? Can I use one backup for multiple domains?

Question 5:
- What are the drawbacks of hosting multiple sites under the same IP? I control all sites under these IPs

Question 6:
- Can somebody recommend a good resource to learn how IP, DNS, nameservers, etc work? Any book, paid website, video... anything ? While there's a ton of info on the web, all is scattered and none seems to be too detailed enough for somebody starting to learn about this.