When you install floating licenses on the Floating server, Max activations field determines on how many servers you are able to install that same license on.
Floating server is designed to be deployed in a secure internal network (e.g. with VPN). Therefore, each user should be a trusted user. If you still want to authenticate users, you can setup an authentication proxy for Floating server.
Floating server can be configured with multiple licenses at the same time. Setting the product allows us to determine to which product that license corresponds to.
You are not able to add multiple licenses for the same product. If you want to change the license, you can remove the old license and add the new one.
Those keys only give you access to the License API, and not to the Management API. License API is a very limited subset of LicenseSpring API, so it is safe to share those keys.