Once you created your account, it will now be time to assign your domain? with the host?. Once this happens, the name servers should be recognized by the host, and will know which account to assign your domain to. DNS propagation could take a couple of days, although I have got lucky sometimes, and had it within hours. Subdomains? will take less time.
If you have multiple domains, it would be important to assign all of the domains to your account. Your host is not going to know which account is to be used until you assign all of the domains you would want for this account. If you decide you need additional domains, you would likely go to the domain registrar? to add more domains. Assuming that your host will give you the correct number of websites, you shouldn’t have any issues with subdomains as long as you stay within your website quota.