Hi EM,
As user of the MongoDB Cloud Manager you can always contact MongoDB using our Cloud Support website.
Unfortunately there is no setting in Cloud Manager which let’s you choose the hostnames. The only way of using your own hostnames is to provision the servers manually and then import them into Cloud Manager. Please have a look at how to Add Existing MongoDB Processes to Cloud Manager.
Alternatively you are able to create the SSL certificates and add DNS aliases to your SSL configuration and certificate files after you created them on *.example.com. You might want to have a look at this CAcert Wiki or apetec.com article in regards to this.
As for the question if you should use elastic IP or not I can’t make any recommendation as this depends entirely on your use case and is maybe best asked in the AWS forum.
Regards,
Markus