While you're perfectly free to purchase a certificate elsewhere, remember that DreamHost offers SSL certificates as well, for $15/year.
Certainly. Certificates that secure subdomains as well are known as "wildcard certificates". They are significantly more expensive than normal SSL certificates, and are not necessary for the vast majority of sites, yours included. (Even if you have multiple subdomains that require secure hosting, it's often still less expensive to purchase multiple normal certificates to cover each of them — the added cost for a wildcard certificate is ridiculously large.)
Outside of some older devices, almost any professionally signed SSL certificate (DreamHost's included) should work just fine with the vast majority of mobile browsers. To the best of my knowledge, all of the current major devices (Android, iOS, current versions of Blackberry and Windows Phone) accept basically the exact same set of certificates as desktop browsers.