Hi akke,
thanks for evaluating Bigace

and here is my input:
1. There are resources that are shared throughout communities and resources that are community specific. Like Dragonslayer said, a Module is communiyt specific. A template is community specific. Plugins are not, they are stored in a global directory for all communities. Sometimes you have extensions that have both, community specific and unspecific resources.
To answer a question you did not ask: Updating bigger extensions sometimes requires you to update it in all communities. Depends on how it was developed and what has changed.
2. Modules are community independent. Modules could use smarty, but most don't, they just "spit out" plain html.
3. The amount of communities is limited by the maximum amount of an integer on your system. I don't believe that you will ever manage to create so many communities. But to be honest, I never tried to create more than 20 communities in a system. It might result in a performance issue running 200 communities, each of them with approx. 100 user and 15000 pages... as I said, I never tried to find out. Technically there is (beside the max amount of an integer in your Mysql) no limit.
4. I do not understand the question regarding Content provider and an invincible community. But there is nothing like a community that is available to all user. Neither in the frontend nor using the PHP Api. You could indeed query the database directly using plain SQL.
But I am not sure if that is what you were asking

5. Again as Dragonslayer said, the "super admin" of each Community sees all other installed Communities within the administration. He cannot access their database/contrent/users/..., but each of the super-admins can install and delete a Community. This is either a bug or a feature by-design - decide for yourself - and hey, I am honest here, I hope you do appreciate that

In most cases you do not want to give away the super-user account. You want to create regular administrator accounts that do not have permissions to access the community administration.
6. You can have as many templates as you want. Each page has its own layout (template) assigned (and inherited from its parent during page creation).
I hope I could help you and would like to welcome you in the Community!
Regards
Kevin