Very interesting questions!
On the first one, yes, the instance may be offline for a couple minutes (2-3min) when upgrading due to the server reset, bootup, etc.
Regarding the second question. Currently the users account manager doesn’t allows to limit the use of an specific plugin, amount of devices or other resources… It will, in the future, but by the moment only we can control this working with the database.
It is important to measure the size of the Server Instance to each use case because, the server RAM and CPU capacity is going to be shared between all the users, so if a lot of them deploys complex NodeRED flows (for example), the server performance may decrease.
Hope you find this useful, please let me know more doubts regarding the new features and I will include it in the documentation or in the deployment FAQ.