Multisite query…

I’ve just created a new site for our customer in a temporary test sub-directory so their existing site can still be live whilst I experiment but I now want to activated multisite. I’ve changed the wp-config.php and gone into Tools>Network Setup but it says…

We recommend you change your siteurl to mainsite.com before enabling the network feature. It will still be possible to visit your site using the www prefix with an address like http://www.mainsite.com but any links will not have the www prefix.

When I go into settings it won’t allow me to remove the www. (not a valid URL) and also mentions in the Network Settings…

Because your install is in a directory, the sites in your WordPress network must use sub-directories. Warning! The main site in a sub-directory install will need to use a modified permalink structure, potentially breaking existing links.

Can I not use Multisite if my site is already in a sub-directory or are they just warning me? I was hoping I could have http://www.mainsite.com/test/website1

Can anyone please shed some light on this?