No, there is no master domain here at dreamhost. It's not a cpanel setup at all. You can just choose to not name any domain up front and just add domains to host later. The default setup for web directories for domains is /home/yourusername/yourdomain.tld, /home/yourusername/yourotherdomain.tld, /home/yourotherusername/subdomain.yetanotherdomain.tld, etc.
Yes, you would do fine by putting your domain name in and changing DNS later. In fact, doing just that is one of the ways of setting up your domain and testing it before you switch over DNS from your old site.
Nope, that's fine too. You can host any number of domains and subdomains in any order you want.
BTW, when you say you want to test out speed, do you mean download speed or speed of execution of sites?
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