Good morning, unfortunately I believe your requestors will be able to see the other people in your org and message them. Testing it right now and it seems a requested could go to messages and when creating a new message, it will show the entire organization member list. At least the way I see it there's no direct way to keep them separate. 

They shouldn’t be able to see other user requests though, so you would still have confidentiality of what is being worked on.

You would have to have the organisations set up for you.

if you are an enterprise customer you could have separate orgs set up for each customer business- they would then become a requestor in that org and will only have access to that one.

We do the same thing but internally, so our quality systems are unable to see our servicing systems

If you’re on a premium sub or higher, would the Request Portal system fit? 

https://watch.getmaintainx.com/videos/7990d6b8121ce1c6f0/creating-request-portals

https://watch.getmaintainx.com/videos/a790d6b9131ae4c32e/using-request-portals

It should allow people to create requests without being added as users, but does have some limitations of its own. Like for example, they can only be linked to one asset - although I suspect you could work around that by creating a “parent” asset for each customer, with all of their assets as children.