Yes. Not only is this "ok" for Dreamhost, Dreamhost's allowing you to host an unlimited number of domains at no additional charge, sharing the storage and bandwidth quotas between them, is one of the things that makes Dreamhost such a good deal! Go for it!
One piece of advice: Though you can set up multiple domains under the same user (and since it so easy to do, many do it that way!), you will gain considerable advantages if you set each client's domain(s) up under their own user. This makes it easier for you to manage "sub-quotas", protects you from having all your clients' sites impacted if a single client "spams" or has a "runaway script", or if you need to give a client direct (ftp, shell, etc.) access to the directory(ies) where their domains' files are stored. While you would then need to login as that user to manage the site, I think the safety and security advantages of doing it this way far outweigh the inconvenience of you managing multiple logins.
One last thing - It is generally much better for the forum if you separate "new" questions into a thread of their own, as it makes it easier for other users to find information that may be relevant to their problem - for instance, no one would think to look under "How do the Promo's work" do learn about hosting multiple domains (which is whay I changed the title on my response)