I’m not sure what it would mean to run MySQL as PHP. I’m pretty sure MySQL is written in C or C++. It is definitely not written using PHP. However, the default MySQL admin tool that DreamHost supports is written in PHP.
Movable Type is written in Perl, not PHP. If you are running on a DreamHost shared server, MT does need to be run as CGI, as opposed to via mod_perl. That will happen as part of the default setup on DreamHost.
Running PHP or Perl programs as CGI does put limits on what they can do, but it won’t affect the functionality of Gallery or MT. The good thing for DreamHost and for the other people on your shared server is that running them as CGI significantly limits the damage that a bug or poor design in one of the programs could cause.