Install DBs other than MySQL on VPS?

I was starting the signup process, and was met with this message:

I’m not quite sure what the intent of this is. Are they saying: “Hey, don’t install MySQL yourself, we’ve got that covered.” Or are they saying, "Only MySQL is allowed, via our service…so please don’t install your own database."
My immediate interest would be to install CouchDB, and maybe also Mongo. Is this permitted?

The intent is mostly the former (“we’ve got MySQL set up for you already”). You can totally set up other databases like CouchDB or MongoDB on a Web VPS yourself; you just won’t get the management that we’ve already put together for MySQL.

Cool. Thanks!