please note, the above I posted is all assuming that:
1. you own a domain called "chelsev.com" (the one you spoke of in the example)
2. you wish to create a nameserver on your machine you will be installing ISPConfig on
3. that you wish to use ns1, ns2.chelsev.com as nameservers for the sites and domains pointed to your machine
If this isn't true, perhaps I misunderstood your question and went of on a litany about 1and1. I have re-read your post and still am not entirely sure.
At the risk of going further into a litany, I think clarifying another point would be helpful.
(Q1) which should i use or is there no differance?
To answer this, you need to set up at least two NS servers for a domain (on any registrar I've used), so you need at least an ns1.whatever.com and an ns2.whatever.com pointed to your machine as "domain host" (not A records). And most registrars demand that they resolve to different IP addresses. Meaning, your machine will need two public IP addresses if you want to use it as DNS server to "the world".