SQL Orion Database - one database ?

Yes, each product uses the same Orion database.  That is part of what makes Solarwinds, well, Solarwinds.  It is a single pane of glass for environment management, and in order to do that it is a single database.

The only thing you need to be careful of is as you add to the environment you may need to beef up your SQL server and/or add additional polling engines, but provided you have SQL server on a VM should be easy enough to expand resources.