Which services ok to turn off when switching to mail services from Google Apps?

Deleting / uninstalling those services will prove difficult, since the Virtualmin packages depend on them. If you remove the packages, the least you'll get is broken dependencies. It's not really recommended to fiddle with those, considering that all the packages will do, when the services are disabled, is take up a little bit of HDD space.

You should rather disable the features and plugins in Virtualmin config that you don't need, and turn off autostarting for those services in Webmin / System / Bootup and Shutdown.

Also, if you acutally succeed in removing the services without breaking the rest, you'll also have removed the configuration etc. of the mail services. I.e. should you decide later that switching to Google was not such a good idea after all (I personally would NEVER-EVER let the Data Kraken handle my email), and you wish to provide mail service again, you'll have to go through the hassle of manually reinstalling AND configuring the mail packages, since you can't just run the Virtualmin installer on a live system.