Hey Triple Moons, thanks for writing in!
Drop down menus are a supported field type for the plugin, so you should be able to see that as an option on your sign up form. One thing you'll want to double-check is that the "update list" button has been clicked in your plugin's settings since that drop down menu was created.
To do that, go to your WP Admin Dashboard > Plugins > Installed Plugins > MailChimp > Settings. Then, click the "update list" button at the top. This ensures that your list fields are in sync with what's available in your plugin's settings.
After doing that, you should be able to see the drop down menu either in the "Merge Variables Included" area or the "Interest Groups" area, depending on whether your drop down menu is setup currently as one or the other.
If you're still not seeing the option available, can you then write an email to mailchimptk[at]gmail.com containing the following info:
-name of the list you're using
-link to the site with the active form, if possible
-screenshot of your merge variables and interest groups area in your plugin settings
In regards to your second point, I can definitely see the desire for a unique identifier for CSS purposes to be able to control, for example, a particular set of interest group choices. As you've mentioned, this is not something that's currently built-in to the plugin. What I can do on my end is to pass this feedback along to the developers for further consideration in a future release.
In the mean time, your best bet to accomplish your intended scenario (using a single list but creating two forms with different field options) is to use the sign up form embed code that's provided in your MailChimp account.
Here's a link that walks through using that:
How can I add my signup form to my website?: http://eepurl.com/gOGN
In using the embed code, you'll have full control over the HTML code that you use on the different pages on your website, which means that you can effectively create different versions of your sign up form with different available options.
If you have any questions, let me know!