The control panel is for all domains (including subdomains) and users. Subdomains work like a normal domain and appear on the menu labeled "domains"
Yup! You just have to keep in mind that this is shared hosting, if one of your sites is really CPU intensive you might get asked to moved to dedicated hosting.
Yup! Bandwidth and disk space will actually grow. If there are any updates to the current packages they will also be applied to current customers (well that's how they've done it since I've been hosting with DreamHost for the last 6 years).
Promo Code: WJD97 - $97.00 off any new DreamHost plan (except month-month payments).