Unique control panel for multiple virtualmin servers ??

How will it work if I add in Cloudmin an existing Virtualmin servers ?

Cloudmin is separate from Virtualmin; it can even be installed on a completely separate machine (and you only need one installation of Cloudmin...it is intended to be a central management UI for many servers). You can install Cloudmin on the same system with Virtualmin, but I usually don't. I like to have it separate so that I can easily keep a replica of it and so that backing it up is really simple. Cloudmin does usually not need a large system if it is not hosting any VMs. In fact, it could run on a small VM itself.

Will it be able to handle it without messing the existing on that virtualmin server ?

Yes, they're separate things. Cloudmin doesn't replace or alter Virtualmin. It manages it from outside. It centralizes some metadata (while leaving the data on the Virtualmin server, as well) to ease things like searching for a specific domain, gathering and displaying resource usage, etc. Think of it more as a helper than as something that takes over the Virtualmin system Virtualmin continues to act exactly as it always did (unless you offload some services, like DNS/databases/mail scanning, in which case there's a few changes).

How does it work for customers ? They can then login on Cloudmin in same way as on Virtualmin ? using same login/pass than previously ?

Yes, there is a Virtualmin multi-login plugin for Cloudmin that allows a central login; it then bounces the user to the right Virtualmin server for their domain. They can continue to login directly, as well. Cloudmin is most for you (or whoever the root admin on all the Virtualmin servers is).

Can I then "create a user account" in Cloudmin that would be able to manage few accounts on different Virtualmin servers ?

Not really, no, though that's a good idea. I'll have to think about how that'd work. Cloudmin doesn't take over the user management for the Virtualmin systems. It might be possible to allow domain creation for a Cloudmin user, which would kinda work out with a similar effect, though user experience might be a little weird.

It's maybe helpful to think of Cloudmin as a meta-manager for Virtualmin, especially for people who have several Virtualmin servers to maintain...not so much for end users (though Cloudmin does have a user account type, it's historically been targeted for creation and management of VMs...I'll have to poke around to see what Virtualmin management capabilities those users have).