As the question says does Galaxy only hosts meteor apps and we need to get mongo from different source or does it provide mongo with it?
No mongo provided, you need to use a third party.
For my galaxy deployment I am using mLabs, MDG recommend Compose. All works fine but it is an extra bit of friction.
Right now I am having a go with NodeChef and its pretty simple so far!
It is cheaper and includes MongoDB hosting. Could be a winner.