When I signed up for an account here I never received a welcome e-mail like some people supposedly did (I signed up over the phone maybe that's why)

I only got one IP address. Should I assume that my other 4 IP are the last number + 4?

Did you submit the support ticket requesting for the justification for the remaining IP addresses ? They are needed for the ARIN justification.

Nope, will do now. Should I submit a normal ticket or is there a special form for this (didn't find one)

Also, should I request for two additional IPs to set up my nameservers or would one more in combination with the one I already have suffice?

There are a variety of ways you can setup your nameservers. Technically, you could run an entire server at SM on 1 ip if you use at least 1 of their nameservers. Otherwise you'll need 2 of your IP's for nameservers. You can either share your main shared IP with a nameserver or not.

Thank you.