The 2 bits you ask:
(I've used 192.168.0.0 as an example subnet)
Your DNS server will be performing forward lookups for your domain.
IN A 192.168.0.10
mail.yourdomain.com. IN A 192.168.0.11
ns0.yourdomain.com. IN A 192.168.0.12
however the reverse lookups for these hosts would be found in the zone file: 0.168.192.IN-ADDR.ARPA which you don't control.
Your ISP will be controlling the above zone file for your IP address. Most ISPs won't modify this for your benefit. The chances are that this would be a name for your connection, e.g. 10.0.168.192-dial-modem.yourisp.com
The forward lookup for ns0.yourdomain.com would resolve to 192.168.0.12 in the above example. However the reverse lookup for the IP would NOT match.
You're not supposed to do this! In reality I've never seen any problems caused by it, but..............
Your zone file should be the same in the primary and secondary nameservers. The secondary obtains the zone information from the primary. Any changes to the primary should be reflected in the secondary in accordance with the "Refresh" time set in the zone file.
Your zone file should contain the names of both DNS servers, e.g.
yourdomain.com. IN NS ns0.yourdomain.com.
yourdomain.com. IN NS nameserver.grancan.com ; example only
When passing this information to your registrar, you would specify the fully qualified names and IP addresses of the primary and secondary name servers.
The hostmaster of the secondary name server would ensure that it was secondary!
Hope that helps,