First, I own all the expansions, DLC etc. But play without Mercenaries as for me they seem to be too disruptive. Do I miss anything new by not playing with Mercs on?
The Bazaar with its mercenary ships, the Arceans and the Torians.
Where can I find out more about commonwealths as to what they are, pro and cons, how and when to form them?
Essentially you create a civilizations that is allied with you and give them planets, their influence is counted as your influence for Influence victory. This is mostly if you have too many planets and don't know what to do with them.
As I understand I earn more money based upon number of tiles I "own". Is this automatic or is there tech I have to research and/or something I have to build?
This is based on a rework of tourism.
Tourism is now based on the amount of tiles you own. This is doubled if these tiles are connected to your home planet and multiplied by Tourism improvements on each colony. The more Influence you have, the more space you control, the more money you will make from Tourism. (this change requires Crusade)