EOS storage seems to support mongoDB out of the box, will it support other SQL and noSQL databases? If so, how do we go about installing postgresSQL on our testnet?
I might be wrong but, as far as I know, mongo is just a plugin that you initialize with your node. It listens for all the activities of the chain like block productions, actions, transactions, accounts creation and simply stores it on a mongodb.
This plugin serves as an example and encourage us to build other ones that fits our necessities. So, yes, based on it, you can always adapt to any other dbs, filesystems etc. it's up to you.
I'm still changing the mongo_plugin for my application, to add filter options, because I want to register only my contracts actions and users, but for now I'm just deleting everything else from mongo each hour, which works fine :)